Add scroll-blocks-on support to devtools layers view

Add scroll-blocks-on support to devtools layers view

Add missing decription of the scroll-blocks-on compositing reason to the
devtools layers view.

Also don't add the compositing reason to the root layer to avoid clutter.
With force-compositing-mode shipped everywhere, this shouldn't effect
whether compositing is actually enabled or not in practice.  However it
should avoid triggering compositing mode unnecessarily in experimental
modes like OOPIF that don't enable it by default.

BUG=347272
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=188962

[Rick Byers, 2015-01-23 15:25:05]

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

[Rick Byers, 2015-01-23 15:26:40]

caseq@ PTAL at this tiny devtools tweak.  I missed updating the layers view when
I added a new compositing reason in https://codereview.chromium.org/784453003.

Thanks,
   Rick

[caseq, 2015-01-23 15:57:33]

lgtm

[Rick Byers, 2015-01-23 16:01:34]

The CQ bit was checked by rbyers@chromium.org

[commit-bot: I haz the power, 2015-01-23 16:02:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/871923005/1

[commit-bot: I haz the power, 2015-01-23 17:56:07]

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

[commit-bot: I haz the power, 2015-01-23 17:56:09]

Try jobs failed on following builders:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/47508)

[Rick Byers, 2015-01-23 21:53:27]

The CQ bit was checked by rbyers@chromium.org

[commit-bot: I haz the power, 2015-01-23 21:54:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/871923005/1

[commit-bot: I haz the power, 2015-01-23 22:44:57]

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

[commit-bot: I haz the power, 2015-01-23 22:44:59]

Failed to commit the patch.
svn: E155010: Commit failed (details follow):
svn: E155010:
'/b/infra_internal/commit_queue/workdir/blink/LayoutTests/paint/theme' is
scheduled for addition, but is missing

[commit-bot: I haz the power, 2015-01-23 22:45:06]

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

[Rick Byers, 2015-01-26 15:58:04]

The CQ bit was checked by rbyers@chromium.org

[commit-bot: I haz the power, 2015-01-26 15:58:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/871923005/1

[commit-bot: I haz the power, 2015-01-26 15:59:26]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=188962