Added more complex profiling metadata.

[cgaebel]

This changes the profiling output from:

_category_                              _mean (ms)_     _median (ms)_   _min (ms)_      _max (ms)_      _bucket size_
CompositingCategory                :          5.3075          4.7763          1.4280         16.6432              12
LayoutPerformCategory              :        284.9125        202.3233        184.6329        967.9776              19
+ LayoutStyleRecalcCategory        :        242.7735        161.5772        146.5625        922.5363              19
+ LayoutDamagePropagateCategory    :          0.6145          0.5888          0.5176          0.8663              19
+ LayoutMainCategory               :         12.3587         11.3791         10.1172         20.1418              19
| + LayoutParallelWarmupCategory   :          0.0008          0.0008          0.0006          0.0014              38
+ LayoutDispListBuildCategory      :         25.6660         23.7513         21.2865         46.7053              19
RenderingDrawingCategory           :         19.6144         19.5144         11.0669         79.0099             150
RenderingCategory                  :        333.6637        349.5627          0.0053        423.6390              13

to:

_category_                          _incremental?_ _iframe?_             _url_              _mean (ms)_     _median (ms)_   _min (ms)_      _max (ms)_      _bucket size_
CompositingCategory                     N/A          N/A                  N/A                        5.7452          5.0217          1.6569         19.6917              11
LayoutPerformCategory                incr reflow    page     http://cnn.com/                       283.5427        201.8654        184.0476       1116.7777              18
LayoutPerformCategory               first reflow    page     http://www.cnn.com/                   325.5460        325.5460        325.5460        325.5460               1
+ LayoutStyleRecalcCategory          incr reflow    page     http://cnn.com/                       239.8541        163.0796        144.7337       1080.4143              18
+ LayoutStyleRecalcCategory         first reflow    page     http://www.cnn.com/                   261.4809        261.4809        261.4809        261.4809               1
+ LayoutDamagePropagateCategory      incr reflow    page     http://cnn.com/                         0.6259          0.6168          0.5312          0.8752              18
+ LayoutDamagePropagateCategory     first reflow    page     http://www.cnn.com/                     0.6078          0.6078          0.6078          0.6078               1
+ LayoutMainCategory                 incr reflow    page     http://cnn.com/                        12.3509         11.9542         10.9876         14.1981              18
+ LayoutMainCategory                first reflow    page     http://www.cnn.com/                    11.3846         11.3846         11.3846         11.3846               1
| + LayoutParallelWarmupCategory     incr reflow    page     http://cnn.com/                         0.0009          0.0009          0.0006          0.0022              36
| + LayoutParallelWarmupCategory    first reflow    page     http://www.cnn.com/                     0.0009          0.0011          0.0007          0.0011               2
+ LayoutDispListBuildCategory        incr reflow    page     http://cnn.com/                        28.0712         24.4887         22.8510         73.7452              18
+ LayoutDispListBuildCategory       first reflow    page     http://www.cnn.com/                    50.3867         50.3867         50.3867         50.3867               1
RenderingDrawingCategory                N/A          N/A                  N/A                       22.2310         21.1877         13.1922         74.4110             138
RenderingCategory                       N/A          N/A                  N/A                      372.2654        390.7910          0.0087        489.5884              12

This patch will not currently build without a checkout of
https://github.com/cgaebel/rust-url:cgaebel-servo-backport listed as an
override in your ~/.cargo/config.

This is blocked on servo/rust-url#29, however, is ready
for review now.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2614

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[cgaebel]

@pcwalton want to be my reviewer? :)

[pcwalton]

Suggestion: Maybe abbreviate the names a little bit? Blink just uses "Style Recalc" and "Layout", which seem good to me.

[pcwalton]

Also a suggestion: "Bucket size" has always been a confusing term. "Events" is probably a better name, since it represents the number of events that were recorded.

[pcwalton]

Other than that, code looks good!