Revert of Remove EnableThreadedCompositing from the browser.

Revert of Remove EnableThreadedCompositing from the browser. (https://codereview.chromium.org/337783002/)

Reason for revert:
Broke virtual/implsidepainting/inspector/timeline, made them use single threaded compositor (and impl side painting).

Original issue's description:
> Remove EnableThreadedCompositing from the browser.
> 
> Now only --disable-threaded-compositing exists, which is only present
> for layout tests. The --enable-threaded-compositing flag moves to
> become a content shell flag only, as it's only used for DumpRenderTree.
> 
> Change any checks in the renderer process for kEnableThreadedCompositing
> to instead be !kDisableThreadedCompositing since that is the only
> flag left.
> 
> Remove the whole threaded compositing thing from about:gpu and
> about:flags.
> 
> R=piman
> BUG=384381, 346363
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=278151

TBR=piman@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=384381, 346363
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=278178

[danakj, 2014-06-18 21:05:34]

Created Revert of Remove EnableThreadedCompositing from the browser.

[danakj, 2014-06-18 21:06:00]

The CQ bit was unchecked by danakj@chromium.org

[danakj, 2014-06-18 21:07:10]

The CQ bit was checked by danakj@chromium.org

[danakj, 2014-06-18 21:07:54]

The CQ bit was unchecked by danakj@chromium.org

[danakj, 2014-06-18 21:07:55]

The CQ bit was checked by danakj@chromium.org

[commit-bot: I haz the power, 2014-06-18 21:08:39]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/danakj@chromium.org/344703002/1

[commit-bot: I haz the power, 2014-06-18 21:11:25]

Change committed as 278178