Implement the HTML5 canvas tainting rules to prevent potential data leakage

Implement the HTML5 canvas tainting rules to prevent potential data leakage

Reviewed by Mitz

Added originClean to HTMLCanvasElement and CanvasPattern
to track whether a canvas (or pattern) is tainted by remote
data.
Use originClean flag to determine whether getImageData should
return, well, image data.

Test: http/tests/security/canvas-remote-read-remote-image.html

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@30869 268f45cc-cd09-0410-ab3c-d52691b4dbfc