Scroll only visual viewport during OSX pinch gesture behind flag.

Scroll only visual viewport during OSX pinch gesture behind flag.

This is part of the experiment to modify the viewport scroll order in some
situations. Namely, during a pinch-zoom gesture, only the visual viewport
should scroll.

The path altered in this patch is used for OSX trackpad pinch-zoom. Touchscreen
pinch-zoom on other platforms is handled in the compositor. This patch matches
the behavior on the CC side for touchscreen pinch-zooms.

BUG=443724
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197855

[bokan, 2015-06-24 15:53:21]

The CQ bit was checked by bokan@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-24 15:53:49]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1202763005/20001

[commit-bot: I haz the power, 2015-06-24 17:54:20]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-24 17:54:22]

Dry run: This issue passed the CQ dry run.

[bokan, 2015-06-25 13:25:54]

bokan@chromium.org changed reviewers:
+ rbyers@chromium.org

[bokan, 2015-06-25 13:25:55]

ptal

[Rick Byers, 2015-06-25 17:08:54]

lgtm

[bokan, 2015-06-25 17:14:47]

The CQ bit was checked by bokan@chromium.org

[commit-bot: I haz the power, 2015-06-25 17:15:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1202763005/20001

[commit-bot: I haz the power, 2015-06-25 19:53:20]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-25 19:53:21]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink (JOB_FAILED,
http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/68335)

[bokan, 2015-06-25 19:54:39]

The CQ bit was checked by bokan@chromium.org

[commit-bot: I haz the power, 2015-06-25 19:54:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1202763005/20001

[commit-bot: I haz the power, 2015-06-25 20:47:23]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197855