Revert 159800 "Move DiscardablePixelRef.cpp/.h to platform/graph..."

> Move DiscardablePixelRef.cpp/.h to platform/graphics/
> 
> BUG=297477
> R=abarth@chromium.org
> 
> Review URL: https://codereview.chromium.org/26433004

TBR=pilgrim@chromium.org

Review URL: https://codereview.chromium.org/27559003

git-svn-id: svn://svn.chromium.org/blink/trunk@159807 bbb929c8-8fbe-4397-9dbb-9b2b20218538