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This CL provides a way for a MediaService client to create a decryptor
which is backed by a CDM or a CdmProxy, also running in the MediaService.

Detailed changes:
- Add CreateDecryptor() in media::mojom::InterfaceFactory
- Plumb CreateDecryptor() through multiple layers to reach
  InterfaceFactoryImpl in MediaService
- In InterfaceFactoryImpl, use the |cdm_id| to find the CdmContextRef,
  and then if a Decryptor is supported, create a MojoDecryptorService.
- Add a dummy D3D11Decryptor implementation.
- Update ClearKeyCdmProxy to support Decryptor.
- Add MediaServiceTest to test this path.

Bug: 806018
Test: media_service_unittests updated.
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: I0eba8aef333cb55ad5180ce1a401f3384c95c9c8
Reviewed-on: https://chromium-review.googlesource.com/1041660Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556946}