Remove the early returns in FrameView::updateCompositingLayersAfterStyleChange.

This function used to do a lot, so the early return was needed
as a performance optimization. Now this function just sets
some dirty bits and sometimes does a bit more work, so we
don't need the early returns.

Also, move enableCompositingModeIfNeeded into the 
CompositingUpdateAfterLayout branch of setNeedsCompositingUpdate.
Otherwise, updateCompositingLayersAfterStyleChange would end up
calling m_compositingReasonFinder.requiresCompositingForScrollableFrame,
which asserts that the RenderView doesn't need layout, which is
obviously not the case inside updateCompositingLayersAfterStyleChange.
Moving it in this branch is correct because enableCompositingModeIfNeeded
only does something if m_rootShouldAlwaysCompositeDirty is true.

The rebaselined test is a progression. I'm pretty sure it's
just a result of different timing in when we do the
compositing update. When there's a root transform layer, the
scrollbar layer should be a child of it instead of the overflow
controls host layer. This behavior was added in
https://src.chromium.org/viewvc/blink?view=rev&revision=167579.
The old behavior points to a bug in the code, i.e.
if we create the scrollbars before creating the root
transform layer then we don't reparent the scrollbars
to that layer.

Review URL: https://codereview.chromium.org/246773010

git-svn-id: svn://svn.chromium.org/blink/trunk@172666 bbb929c8-8fbe-4397-9dbb-9b2b20218538