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This removes the hardcoded setup of the decrypting audio
and video decoders out of the DecoderSelector and into the
standard decoder list.

This functions similarly to the old behavior, but instead of
knowing except for the following differences:
- All decoders are checked for encrypted support first now.
- If no decoder succeeds in initialization with an encrypted
configuration, we'll try creating a DecryptingDemuxerStream,
if this succeeds we'll check the decoder list again, if it
fails, we'll abort as before.

Mainly this implies that falling back to decrypt-only will
take slightly longer than before. This also adds back support
for EME to use GpuMemoryBuffers.

BUG=801245
TEST=updated lots of unittests...

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I578809aeebaa59d81f65a5be2f2f1f76713d0f68
Reviewed-on: https://chromium-review.googlesource.com/952207Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542323}