Implement enableFastMobileScrolling web api.

Implement enableFastMobileScrolling web api.

The effort needs to be done in 3 steps:
1. Create the runtime flag interface in blink;
2. Enable the flag for Android from chromium even it does not do anything in blink;
3. Replace compile time guard with runtime flag in blink and remove ENABLE_FAST_MOBILE_SCROLLING in gyp file.

This is the 1st set of changes.

BUG=381391
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175621

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175761

[tkent, 2014-06-05 22:59:04]

lgtm

[commit-bot: I haz the power, 2014-06-05 22:59:19]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/c.shu@samsung.com/321473002/20001

[commit-bot: I haz the power, 2014-06-06 01:32:21]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/win_blink_rel/builds/10874)

[commit-bot: I haz the power, 2014-06-06 02:10:28]

Change committed as 175621

[dcheng, 2014-06-06 02:21:19]

A revert of this CL has been created in
https://codereview.chromium.org/317313002/ by dcheng@chromium.org.

The reason for reverting is: Broke the build..

[c.shu, 2014-06-06 14:36:36]

On 2014/06/06 02:21:19, dcheng wrote:
> A revert of this CL has been created in
> https://codereview.chromium.org/317313002/ by mailto:dcheng@chromium.org.
> 
> The reason for reverting is: Broke the build..

It seems the problem was from this bug:
https://code.google.com/p/chromium/issues/detail?id=381111
Shall we try to restore this CL? Thanks.

[c.shu, 2014-06-06 17:44:41]

The CQ bit was checked by c.shu@samsung.com

[commit-bot: I haz the power, 2014-06-06 17:45:05]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/c.shu@samsung.com/321473002/20001

[commit-bot: I haz the power, 2014-06-06 17:46:20]

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

[commit-bot: I haz the power, 2014-06-06 17:46:21]

Could not make sense out of svn commit message.

[c.shu, 2014-06-08 23:30:21]

The CQ bit was checked by c.shu@samsung.com

[commit-bot: I haz the power, 2014-06-08 23:31:12]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/c.shu@samsung.com/321473002/20001

[commit-bot: I haz the power, 2014-06-08 23:31:32]

Change committed as 175761