Retire the WebContextMenuData::isImageBlocked as we already switched to use WebContextMenuData::hasImageContents instead.

See related changes in https://codereview.chromium.org/22938006 and https://codereview.chromium.org/23200006.


BUG=272076
Test=None

Review URL: https://chromiumcodereview.appspot.com/23037018

git-svn-id: svn://svn.chromium.org/blink/trunk@156673 bbb929c8-8fbe-4397-9dbb-9b2b20218538