Revert of Remove WebViewImpl::setIsAcceleratedCompositingActive

Revert of Remove WebViewImpl::setIsAcceleratedCompositingActive (patchset #5 id:80001 of https://codereview.chromium.org/1078473002/)

Reason for revert:
Causes blank pages on load sometimes

Original issue's description:
> Remove WebViewImpl::setIsAcceleratedCompositingActive
> 
> This function no longer makes sense now that WebViewImpl is either
> always or never composited.  The setup that is done when a new
> root layer is set is now done whenever those settings are changed,
> as the WebLayerTreeView is always there to store those values.
> 
> There's nothing pressing here to change this; it's just previous
> cruft that never got removed when force compositing mode got turned
> on.  The new code seems a lot cleaner.
> 
> BUG=474818
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=193897

TBR=pdr@chromium.org
BUG=474818, 479270, 478578, 478686
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194244

[enne (OOO), 2015-04-22 18:27:22]

Created Revert of Remove WebViewImpl::setIsAcceleratedCompositingActive

[enne (OOO), 2015-04-22 18:29:40]

The CQ bit was checked by enne@chromium.org

[commit-bot: I haz the power, 2015-04-22 18:30:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1094453004/1

[commit-bot: I haz the power, 2015-04-22 23:07:08]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194244