Introduce CSSPixel as a replacement for ViewportPx and PagePx.

[glennw]

This change is 

[highfive]

Heads up! This PR modifies the following files:

• @bholley: components/style/servo/media_queries.rs, components/style/viewport.rs
• @jgraham: components/webdriver_server/lib.rs
• @emilio: components/style/servo/media_queries.rs, components/layout/display_list_builder.rs, components/style/viewport.rs
• @fitzgen: components/script/dom/document.rs, components/script_traits/script_msg.rs, components/script_traits/script_msg.rs, components/script_traits/lib.rs, components/script_traits/lib.rs, components/script/dom/window.rs, components/script/dom/mediaquerylist.rs
• @KiChjang: components/script/dom/document.rs, components/script_traits/script_msg.rs, components/script_traits/script_msg.rs, components/script_traits/lib.rs, components/script_traits/lib.rs, components/script/dom/window.rs, components/script/dom/mediaquerylist.rs
• @asajeffrey: components/constellation/pipeline.rs, components/webdriver_server/lib.rs, components/constellation/constellation.rs

[highfive]

Warning

• These commits modify style, layout, and script code, but no tests are modified. Please consider adding a test!

[glennw]

This change deserves some explanation.

This PR is (sort of) a WIP. I want to get some feedback on whether
this seems like a viable / correct approach to handling both
page and pinch zoom. Assuming it does, we can either merge this
after review (which would be my preference) or wait until the WR
pinch zoom functionality is ready before merging this part.

The idea is to move pinch-zoom to be handled by WR as (almost)
an internal detail. There would be an API to control the pinch zoom
factor, but apart from that WR would handle pinch-zoom internally.

From what I can tell of the specifications and browser behaviour,
this makes sense - but it's quite possible I've missed an important
detail.

Given the assumptions above, the Servo code now only needs to deal
with three coordinate spaces, rather than four. Specifically, we now
would have:
Device pixels -> Device independent pixels (hi-dpi factor) -> CSS pixels (page zoom factor)

This PR doesn't break any existing functionality, since pinch zoom
doesn't currently work at all. Page zoom is also currently broken
although that's orthogonal to this PR.

[glennw]

r? @mbrubeck

[glennw]

Rebased and fixed tidy error.

[jdm]

Style unit tests need updating.

[glennw]

Thanks, updated the unit tests.

[mbrubeck]

Why a new type instead of ScaleFactor?

[mbrubeck]

@bors-servo r+

Curious about the answer to the above question, but no changes required.

[bors-servo]

📌 Commit 30ff2f8 has been approved by mbrubeck

[bors-servo]

⌛ Testing commit 30ff2f8 with merge 1d13e6a...

[bors-servo]

☀️ Test successful - android, arm32, arm64, linux-dev, linux-rel-css, linux-rel-wpt, mac-dev-unit, mac-rel-css, mac-rel-wpt1, mac-rel-wpt2, windows-gnu-dev, windows-msvc-dev
Approved by: mbrubeck
Pushing 1d13e6a to master...