Bug 1107593 - Part 1: unit testing of the cover art in m4a files.
Bug 1107593 - Part 2: metadata field is 'picture' not 'image'. findMoovAtom() return a promise.
Merge pull request #26627 from hfiguiere/bug1107593-m4a-parsing-fixes

Bug 1107593 - m4a parsing fixes. r=squib