cc: Disallow scroll offset animations when impl-scrolling isn't supported
SingleThreadProxy doesn't support directly changing scroll offsets on
the impl layer tree (since it doesn't send scroll deltas from the impl
side back to the main-thread side). This means that scroll offset
animations should be disallowed by cc when using SingleThreadProxy.
BUG=243871
Review URL: https://codereview.chromium.org/
368883003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281140 0039d316-1c4b-4281-b951-d872f2087c98