[CSS Exclusions] Minimal support for using an image to define a shape

This is a first small step towards supporting CSS shapes defined by an image URL and a alpha channel threshold. To keep the patch as small as possible, there are many limitations and remaining work items. See https://code.google.com/p/chromium/issues/detail?id=252737 for the current list.

BUG=252752

Review URL: https://chromiumcodereview.appspot.com/17379023

git-svn-id: svn://svn.chromium.org/blink/trunk@156057 bbb929c8-8fbe-4397-9dbb-9b2b20218538