[chromium] Build fix.

Unreviewed build fix. Remove the use of SkMutex from DiscardablePixelRef.

* platform/graphics/chromium/DiscardablePixelRef.cpp:
(WebCore::DiscardablePixelRefAllocator::allocPixelRef):
(WebCore::DiscardablePixelRef::DiscardablePixelRef):
* platform/graphics/chromium/DiscardablePixelRef.h:
(DiscardablePixelRef):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141033 268f45cc-cd09-0410-ab3c-d52691b4dbfc