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This CL fixes the host process so that the client can successfully
select a different codec after SDP offer/answer exchange is restarted
during an active connection. This comes in 2 parts:

1. The current encoder in WebrtcVideoStream is reset, so that
captured frames are routed to the new encoder.

2. The WebrtcVideoEncoderSelector logic is slightly tweaked so that a
call to SetPreferredCodec() resets the cycle of encoders returned by
repeated calls to CreateEncoder(). This is done so that the correct
encoder is returned after WebrtcVideoStream changes the preferred codec.
This tweak does not break any existing unittests.

This CL fixes crashes which occur when the host tries to use a different
encoder than what WebRTC created via the WebrtcDummyVideoEncoderFactory
class. These crashes can only be triggered when SDP (or ICE) is
restarted via the recently-implemented control messages, which are not
supported in older versions of the host.

Bug: 1113499
Change-Id: Idd1e388252c4359e1edc62ac4a51d71be0ae77cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353672
Auto-Submit: Lambros Lambrou <lambroslambrou@chromium.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797705}