Automatic update from web-platform-tests[css-layout-api] Populate children, and pass into layout() function 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-commits: 41ab079dfcde76aebbde5660fe3e417447e61f80 wpt-pr: 9680 wpt-commits: 41ab079dfcde76aebbde5660fe3e417447e61f80 wpt-pr: 9680