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All players/decoders handling encrypted media should implement the logic
to suspend playback when the decryption key is not available, and then
resume playback when a new usable decryption key is added. Hence,
there's a common need for the players/decoders to be notified when such
an event (new usable key) happens.

Since CdmContext is the common interface bridging players/decoders and
the CDM, this CL adds RegisterNewKeyCB() to CdmContext.

To facilitate the implementation of RegisterNewKeyCB() in various CDMs
and CdmProxies, CallbackRegistry is also introduced, which can manage
callback registration, notification and unregistration. Note that
there's base::CallbackList which is similar, but that class is not as
simple and is not thread-safe.

This CL also updates D3D11CdmProxy to use CallbackRegistry to manage
new key callbacks. D3D11VideoDecoderImpl is also updated to be able to
register new key callbacks.

Bug: 821288
Test: Covered by existing tests.
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: I0380f934efb63d3b3c893d909e0d78f5f8e082f4
Reviewed-on: https://chromium-review.googlesource.com/1014245
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551468}