Initialized ImageData with transparent black when constructed with no data

This behavior is already tested, and the test already passes (by coincidence?)
even though we were using creteUninitialize to allocate the data array.

BUG=361694

Review URL: https://codereview.chromium.org/230823005

git-svn-id: svn://svn.chromium.org/blink/trunk@171170 bbb929c8-8fbe-4397-9dbb-9b2b20218538