media::CdmProxy interface

- C++ version of the CdmProxy interface.

Bug: 785563
Change-Id: I64ca5f478c19505361eb7f7eedde3f5c208db488
Reviewed-on: https://chromium-review.googlesource.com/773567


Commit-Queue: Rintaro Kuroiwa <rkuroiwa@chromium.org>
Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518295}