Like video_decoder.cc, a call to Encode that returns
WEBRTC_VIDEO_CODEC_FALLBACK_SOFTWARE will trigger an attempted fallback
to a built-in software encoder. Initialization information, along with
any rate and channel parameter info, will be replayed on the software
encoder and then the frame (that cause the fallback) will be immediately
replayed for the software encoder.
Also modified the existing behavior to Release() the "real" encoder even
if a fallback encoder exists. That seems like the correct behavior.
BUG=webrtc:2920
Review URL: https://codereview.webrtc.org/1328863002
Cr-Commit-Position: refs/heads/master@{#10368}
b1ce663d16