Add entry to about:flags for scrollTopLeftInterop

Add entry to about:flags for scrollTopLeftInterop

This adds easy UI for developers to test Chrome's new standard-compliant
scrollTop behavior before we make it the default (and for people to
easily check whether turning it off resolves issues shortly after it
becomes the default behavior).

BUG=157855
Committed: https://crrev.com/3f362dc40ef117a8265e3a6126dd6252fd2ef1c5
Cr-Commit-Position: refs/heads/master@{#328772}

[Rick Byers, 2015-05-07 02:47:08]

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

[Rick Byers, 2015-05-07 02:47:08]

Tim, PTAL
Opera DevRel suggests (and I agree) that we should have an entry for this in
about:flags during the transition phase to enable easy testing (especially on
Android and ChromeOS where it's hard to set command line flags).

Note that this is apparently the first time that an about:flags entry directly
uses --enable-blink-features.  I considered creating a special-purpose command
line flag, but this is so much simpler.  One downside is that with this you get
the "you are using an unsupported command line flag, security and stability will
suffer" banner but I think that's fine for developer testing purposes.

[tdresser, 2015-05-07 13:15:01]

On 2015/05/07 02:47:08, Rick Byers wrote:
> Tim, PTAL
> Opera DevRel suggests (and I agree) that we should have an entry for this in
> about:flags during the transition phase to enable easy testing (especially on
> Android and ChromeOS where it's hard to set command line flags).
> 
> Note that this is apparently the first time that an about:flags entry directly
> uses --enable-blink-features.  I considered creating a special-purpose command
> line flag, but this is so much simpler.  One downside is that with this you
get
> the "you are using an unsupported command line flag, security and stability
will
> suffer" banner but I think that's fine for developer testing purposes.

LGTM.

[Rick Byers, 2015-05-07 13:31:38]

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

[Rick Byers, 2015-05-07 13:31:39]

+asvitkine@ for histograms OWNERS

[Alexei Svitkine (slow), 2015-05-07 15:01:01]

lgtm

[Rick Byers, 2015-05-07 15:27:00]

The CQ bit was checked by rbyers@chromium.org

[commit-bot: I haz the power, 2015-05-07 15:27:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1130073002/1

[commit-bot: I haz the power, 2015-05-07 16:20:43]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-07 16:21:36]

Patchset 1 (id:??) landed as
https://crrev.com/3f362dc40ef117a8265e3a6126dd6252fd2ef1c5
Cr-Commit-Position: refs/heads/master@{#328772}