Make rootScroller the layout viewport.
In order to properly support pinch zoom and other viewport scrolling actions, we
need to promote the rootScroller element's ScrollableArea to be the layout
viewport. This is primarily accomplished by replacing the layout viewport in
RootFrameViewport.
I've moved the ViewportScrollCallback creation to happen at Document attachment
time. This is necessary since the FrameView may not have fully initialized its
ScrollableArea until this happens. Notably, this is needed for
root-layer-scrolls since the ScrollableArea for a FrameView will be the
LayoutView's PaintLayerScrollableArea which won't be initialized until
attachment.
I've also updated the set-root-scroller test to break up the one big test and
replace it with many small ones.
BUG=505516
Review-Url: https://codereview.chromium.org/2128553002
Cr-Commit-Position: refs/heads/master@{#404250}
11 files changed
