Draw methods for ImageBitmap.

We store ImageBitmaps with the top-left corner of the bitmap at (0,0)
and do some math in the draw command to draw it correctly to the canvas.

We are not actually inserting "transparent black pixels" for overcropped
regions as the spec specifies.

BUG=166658

Review URL: https://chromiumcodereview.appspot.com/18518004

git-svn-id: svn://svn.chromium.org/blink/trunk@154185 bbb929c8-8fbe-4397-9dbb-9b2b20218538