Move WebLayerTreeView::SetRootLayer to WebWidgetClient. (1/n)

The WebLayerTreeView is a shortcut past the WebWidgetClient that is
not needed now that we always composite. This works toward eliminating
this extra API layer

More importantly this helps us Close/Restart the WebWidget parts of
WebViewImpl by not requiring plumbing 2 pointers with slightly different
lifetimes (WebLayerTreeView is created inside the Init of the
WebWidgetClient). This will help avoid a bunch of complexity in
creating WebFrameWidgets and other WebWidgets.

TBR=dcheng@chromium.org

Bug: 912193
Change-Id: Id9b496255f7f8bb67405f779903b6ba1826c3716
Reviewed-on: https://chromium-review.googlesource.com/c/1456205
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630414}
15 files changed