Make RootScroller set the outer viewport scroll layer in the compositor
This CL makes the outer viewport scroll layer in the compositor match the
RootScroller element's scroll layer. It adds a callback in RootScroller
that registers the viewport's layers each time compositing in the
FrameView is updated. The WebViewImpl::registerViewportLayersWithCompositor
method queries RootScroller for the layer to use for the outer viewport
scroll layer.
Without enabling the RootScroller API, this will always fall back to the
FrameView's scroll layer so it'll only have an effect with experimental
flags enabled.
BUG=505516
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel
Review-Url: https://codereview.chromium.org/2113483002
Cr-Commit-Position: refs/heads/master@{#404890}
9 files changed
