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This CL changes DecoderStream to maintain an instance of DecoderSelector
continually. This enabled DecoderSelector to maintain
DecryptingDemuxerStream state and the blacklist internally.

An immediate consequence is that DecoderSelector can now always try the
full list of potential decoders each time that selection is triggered.
This fixes the GPU->GPU changeType() transition, and provides a
foundation to build GPU fallforward on top of.

Another consequence is that fallback can occur more than once for a
stream. It is no longer possible to loop forever doing fallbacks, so this
is a robustness improvement in addition to being a requirement for
changeType().

Bug: 877673
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I76d1bed1c914e1ec58a6653d7200fbb67f70971a
Reviewed-on: https://chromium-review.googlesource.com/1188978
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587908}