Lock touchscreen flings to the first layer scrolled by the fling

The current conditional fling scroll bubbling logic in LayerTreeHostImpl is
less than perfect, and leads to scenarios where fling scrolls continue bubbling
after the user would expect. Instead, simply lock the fling scroll updates to
the first layer that scrolls after the fling begins. This allows both seamless
flings after touch-scrolling across layers and finer control over when such
flings terminate.

BUG=328217

Review URL: https://codereview.chromium.org/114723005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242186 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242186