Fixes <http://webkit.org/b/32745>.

Reviewed by Ada Chan.

* Interfaces/IWebPreferencesPrivate.idl: Added
[set]AcceleratedCompositingEnabled.

* Interfaces/WebKit.idl: Touched to force a build.

* WebPreferenceKeysPrivate.h: Added
WebKitAcceleratedCompositingEnabledPreferenceKey.

* WebPreferences.cpp:
(WebPreferences::initializeDefaultSettings): Make accelerated
compositing be on by default.
(WebPreferences::setAcceleratedCompositingEnabled): Store the new
value.
(WebPreferences::acceleratedCompositingEnabled): If accelerated
compositing isn't available, return false. Otherwise, return the value
stored in preferences.

* WebPreferences.h: Added [set]AcceleratedCompositingAvailable.

* WebView.cpp:
(WebView::notifyPreferencesChanged): Just pass the value from
WebPreferences on down.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@52442 268f45cc-cd09-0410-ab3c-d52691b4dbfc