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We need to attach the document animation timeline to the animation host
to make animations work. In pre-CompositeAfterPaint, this is done from
PaintLayerCompositor.

For CompositeAfterPaint, attach animation timeline from
Document::Initialize() and WebLocalFrameImpl::SetFrameWidget().

Bug: 702353
Change-Id: I5e19add5196e5b52ebcade42b58e336051b111e6
Reviewed-on: https://chromium-review.googlesource.com/c/1368994Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615554}