[PeerConnection] Fire signalingstatechange event at the right time

Prior to this CL, the event was fired before the transceiver state was
updated, in contradiction with the spec, which says it should be fired
after the transceiver state is updated.

Bug: 920200
Change-Id: I757cc0161a5da4888cd628619180e24a54dc732b
Reviewed-on: https://chromium-review.googlesource.com/c/1458203
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634609}
3 files changed