Bug 1610009 [wpt PR 21242] - Implement new effect behavior for the root element....

Bug 1610009 [wpt PR 21242] - Implement new effect behavior for the root element., a=testonly

Automatic update from web-platform-tests
Implement new effect behavior for the root element.

Intent-to-ship approval:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/4DC56FN3paU

Changes are:
1. The background of the root element paints into the property tree
state that includes the local border box effect state of the root
element.
2. If #1 is happening, paint a separate backdrop in the ContentsProperties
state of the LayoutView, to allow effects, clips and transforms of the
root element to apply via PaintChunksToCcLayer.

Bug: 898293,988446,1029170,711955

Change-Id: I751e3f9c23d0b06a31cf1b13c3853180bc7ac7e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1979258
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733661}

--

wpt-commits: 4209d7961bab7dce31baab0918fa9f6dcd74bd8d
wpt-pr: 21242