Bug 1108059 - Enable mp4, not webm, for MediaSource on MacOS X. r=ajones, a=sledru

This makes the setting the same as on Windows. This will reduce
the amount of testing we get for WebM, since only Linux will be
left with that enabled, but reduces the exposed code surface while
we're trying to stabilize things enough to ship support.
Bug 1097823 - Move NotDecodedReason from RequestSampleCallback to MediaDecoderReader. r=cpearce, a=sledru
Bug 1097823 - Implement MediaTaskQueue::ForceDispatch. r=cpearce, a=sledru

This is necessary to have strong guarantees that promises will be resolved.
While we're flushing the task queue, normal dispatch starts to fail,
meaning that we can't dispatch promise resolution. We have 3 options to handle
this:

(A) Never respond to the promise.
(B) Invoke the Resolve/Reject callback synchronously if dispatch fails.
(C) Prevent dispatch from failing.

(C) seems like the option least likely to violate invariants if we can get away
with it. Promise resolution is unlikely to be a heavyweight task in the way that
a decode task might be, so this should hopefully be ok.

Note that this still doesn't help for bonafide task queue shutdown. It's up to
consumers to tear down their MediaPromiseHolders before the task queues are shut
down.
Bug 1097823 - Implement Media Promises. r=cpearce, a=sledru
Bug 1101969 - Disable pinning on media.mozilla.com. r=keeler, a=sledru
Bug 1105541 - Part 1: Use a light text color for tab titles with a dark LWT on new tablet. r=mhaigh, a=sledru
Bug 1105541 - Part 2: Add alpha to pressed state of tabs with LWT on new tablet. r=mhaigh, a=sledru
Bug 1105541 - Part 3: Change add tab button to a dark color on light themes on new tablet. r=mhaigh, a=sledru