ImageSource misc cleanup

Remove unused includes, simplify and clarify a comment, format the
frameBytesAtIndex() code like the rest of the code in this file.

BUG=None

Review URL: https://codereview.chromium.org/485623002

git-svn-id: svn://svn.chromium.org/blink/trunk@180434 bbb929c8-8fbe-4397-9dbb-9b2b20218538