GitLab

This CL fixes the scroll chaining behavior for elements that aren't descendants
of the root scroller element. This can only happen in the non-document root
scroller proposal.

On the main thread scrolling path, we were previously assuming that all scrolls
would chain through the effective root scroller. This CL fixes the terminating
condition as well as removes an invalid DCHECK.

On the compositor side, the chaining mechanisms would previously use the inner
viewport scroll layer to designate viewport scrolling. Further on, we would
explicitly check for the inner viewport layer and scroll using cc::Viewport.
When scrolling a non-descendant of the root scroller, we chain up to the inner
viewport scroll layer without going through the outer viewport. Scrolling
cc::Viewport causes scrolling in the outer viewport though so we would scroll
it inadvertaintly.  I've made changes so that we use the outer viewport to
designate cc::Viewport scrolling. Scrolls will terminate at the outer viewport
scroll layer, but they don't have to chain through it.

Unrelated, but I also removed LayerImpl::ApplyScroll as it's unused.

BUG=505516
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2365793002
Cr-Commit-Position: refs/heads/master@{#420763}