[chromium] Add WebWidget API for accessing the current WebCompositor

[chromium] Add WebWidget API for accessing the current WebCompositor
https://bugs.webkit.org/show_bug.cgi?id=69181

Reviewed by Darin Fisher.

Add new WebWidgetClient::did(Activate|Deactivate)Compositor calls intended to replace
didActivateAccleratedCompositing(bool) so that the enable call can be parameterized.

Add a WebCompositor identifier parameter to didEnableAcceleratedCompositing that can be used on the compositor
thread to get access to a WebCompositor pointer.

* public/WebWidget.h:
(WebKit::WebWidget::compositor):
* src/WebViewImpl.cpp:
(WebKit::WebViewImpl::compositor):
(WebKit::WebViewImpl::setIsAcceleratedCompositingActive):
* src/WebViewImpl.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@96529 268f45cc-cd09-0410-ab3c-d52691b4dbfc