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With this change, onsignalingstatechange always fires before the
setLocalDescription's promise resolves.

Before this change, signaling state changes were surfaced with callbacks
firing separately from the callback that setLocalDescription or
setRemoteDescription had completed. As a result, the result of a SLD/SRD
operation was surfaced in two PostTasks instead of one. This could lead
to flaky behavior where JavaScript task execeution cycles may or may not
execute between PostTasks which lead to undefined order. Also when the
signalingstatechange was surfaced to the JS thread the firing of the
event was delayed by a task execution cycle due to
ScheduleDispatchEvent.
- If both PostTasks were handled in the same cycle, the event firing
  would be scheduled and the promise resolved, such that the promise's
  then() is invoked just before the event fires (scheduled after
  promise). This is NOT in accordance with the spec.
- If onsignalingstatechange's PostTask was handled in its own cycle,
  the event would be scheduled such that by the time the
  setLocalDescription's callback's PostTask occurs resolving the
  promise, the event would fire before the promise's then(). This IS
  in accordance with the spec.

With this change, the onsignalingstatechange event is fired without
delay, and it is processed in by the same observer that handles the
SLD/SRD callback, so the order is always guaranteed to be the same.

In addition, this CL removes the usage of
WebRTCPeerConnectionHandlerClient::SignalingState and relies on the
webrtc::PeerConnectionInterface::SignalingState enum directly. This
is part of the Onion Souping work (crbug.com/787254).

Bug: 848768, 787254
Change-Id: I7bc10439c44c6ff6beab0289975295ff2e051b9c
Reviewed-on: https://chromium-review.googlesource.com/1085302
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565291}