[Cast] Size-Adaptable platform video encoders.

Provides a proxy VideoEncoder implementation that wraps
ExternalVideoEncoder or H264VTEncoder to handle frame size changes.
When the frame size changes, the proxy 1) waits for current encoder
instance to finish encoding any frames in-progress, then 2) destroys the
encoder, and 3) finally re-creates/initializes a new encoder instance.
During this process, frames provided via InsertRawVideoFrame() are
simply dropped.

Merged external_encoder_unittest.cc and video_encoder_impl_unittest.cc
into a single video_encoder_unittest.cc that tests *all* cast
VideoEncoder implementations.  Made a number of fixes to allow testing
of the H264VTEncoder on desktop Macs and the trybots.

BUG=451277

Review URL: https://codereview.chromium.org/906403006

Cr-Commit-Position: refs/heads/master@{#315723}

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