[chromium] Add null pointer check to animatePageScale

[chromium] Add null pointer check to animatePageScale
https://bugs.webkit.org/show_bug.cgi?id=83940

Patch by Alexandre Elias <aelias@google.com> on 2012-04-13
Reviewed by James Robinson.

Add null pointer check to startPageScaleAnimation.

If the root scroll layer goes away in the middle of a page scale
animation, there can be a null pointer access here.

No new tests.

* platform/graphics/chromium/cc/CCLayerTreeHostImpl.cpp:
(WebCore::CCLayerTreeHostImpl::animatePageScale):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114199 268f45cc-cd09-0410-ab3c-d52691b4dbfc