Add a flag for enabling/disabling the new accelerated scrolling path

Relanding. This patch had previously been reverted because opting into the new path could run us into bugs. All known bugs have been resolved, so this should be safe to land.

We now have two accelerated scrolling paths. One accelerates scrolling whenever
it's safe to promote the overflow scrolling div to a stacking container /
containing block and not break stacking or clipping.

The new path, the 'universal' path, enables accelerated scrolling in those cases
where the old path would not opt-in.

Eventually, we'd like to eliminate the old path, but this will require layer
squashing (the new path can result in many composited layers that ought to be
combined -- i.e., a 'layer explosion'). Until these paths are mature, we need to
be able to enable and disable them both, so we have two separate sets of flags.

Once we've eliminated the old path, we can replace both with
--[enable/disable]-accelerated-overflow-scroll (and eventually get rid of both
sets of flags).

BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=228187

R=darin@chromium.org, hartmanng@chromium.org, shawnsingh@chromium.org

Review URL: https://codereview.chromium.org/26809004

git-svn-id: http://src.chromium.org/svn/trunk/src/build@229421 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

ios/grit_whitelist.txt

@@ -625,6 +625,8 @@ IDS_FLAGS_FORCE_FULLSCREEN_APP_DESCRIPTION
 IDS_FLAGS_FORCE_FULLSCREEN_APP_NAME
 IDS_FLAGS_FORCE_HIGH_DPI_DESCRIPTION
 IDS_FLAGS_FORCE_HIGH_DPI_NAME
+IDS_FLAGS_FORCE_UNIVERSAL_ACCELERATED_OVERFLOW_SCROLL_MODE_DESCRIPTION
+IDS_FLAGS_FORCE_UNIVERSAL_ACCELERATED_OVERFLOW_SCROLL_MODE_NAME
 IDS_FLAGS_FULL_HISTORY_SYNC_DESCRIPTION
 IDS_FLAGS_FULL_HISTORY_SYNC_NAME
 IDS_FLAGS_HIDE_LAUNCHER_ALIGNMENT_MENU_DESCRIPTION