[css-layout-api] Populate children, and pass into layout() function

[chromium-wpt-export-bot]

This introduces the LayoutChild API surface, (which only has a styleMap
accessor currently).

Each LayoutBox which is a child of a LayoutCustom will hold onto a
LayoutChild object, acting as the script interface for the web
developer.

Bug: 726125
Change-Id: Iecdb01a38c0712dcda063b485c4207bde59f3995
Reviewed-on: https://chromium-review.googlesource.com/933092
Commit-Queue: Ian Kilpatrick ikilpatrick@chromium.org
Reviewed-by: Morten Stenshorne mstensho@chromium.org
Reviewed-by: Rune Lillesveen futhark@chromium.org
Reviewed-by: Koji Ishii kojii@chromium.org
Cr-Commit-Position: refs/heads/master@{#539928}

[wpt-pr-bot]

Already reviewed downstream.

[w3c-bots]

Build ERRORED

Started: 2018-03-01 04:53:12
Finished: 2018-03-01 05:28:05

Failing Jobs

• firefox:nightly
• safari:11.0
• MicrosoftEdge:16.16299

View more information about this build on:

• WPT PR Status
• TravisCI

[foolip]

Firefox job was failing and Travis didn't even have the log, so I rebased this on master to give it another go. (Restart seems to not work.)

[foolip]

This time there was a log:
https://api.travis-ci.org/v3/job/347597990/log.txt

There were 72 affected tests, and by searching for some of the names of the first and last affect tests, it seems like we were in the middle of the 5th run when it broke:

Log reached capacity (3145728 bytes); output disabled.


No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

The build has been terminated

If "output disabled" means what it seems, then I think that #9657 is the culprit here. Will comment there and force merge this.