Make WebrtcVideoEncoder asynchronous.

Previously WebrtcVideoEncoder interface was synchronous, so it was
incompatible with HW encoders. Refactored it to make it asynchronous.

Change-Id: I1b0f3ce6dda657ecddb4ac84cf8132a22b473308
Reviewed-on: https://chromium-review.googlesource.com/541122
Reviewed-by: Scott Nichols <[email protected]>
Commit-Queue: Sergey Ulanov <[email protected]>
Cr-Commit-Position: refs/heads/master@{#481297}
8 files changed