Add ability to instantiate VideoEncoderSoftwareFallback in VP tests.

* Split TestConfig::hw_codec into hw_encoder and hw_decoder.
* Add TestConfig::sw_fallback_encoder.

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/3009963002
Cr-Original-Commit-Position: refs/heads/master@{#19710}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d635e5b8ed2e77d4dac6b08d0f33f6fba466632e
7 files changed