Blob creation methods for ImageBitmap.

Error handling for non-image blobs will be done in a future patch, once
promises have been implemented.
 
BUG=260984, 166658

Review URL: https://chromiumcodereview.appspot.com/20748002

git-svn-id: svn://svn.chromium.org/blink/trunk@156101 bbb929c8-8fbe-4397-9dbb-9b2b20218538