brokenImage() is now a static member of ImageResource class

Changed the method signature and the function calls.
No new testcases needed.

BUG=

Review URL: https://chromiumcodereview.appspot.com/23083003

git-svn-id: svn://svn.chromium.org/blink/trunk@156407 bbb929c8-8fbe-4397-9dbb-9b2b20218538