media: Add Mojo CDM File I/O implementation

Adds a cdm::FileIO implementation that can be used by mojo CDMs to read and
write files. Files are obtained from the mojo CdmStorage interface.

BUG=479923,776911
TEST=updated browser_test and new unit tests pass

Change-Id: Ib150058407d6db1d0ba2cabf4d5eb3f4a051232c
Reviewed-on: https://chromium-review.googlesource.com/706211
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Bo <[email protected]>
Reviewed-by: Xiaohan Wang <[email protected]>
Commit-Queue: John Rummell <[email protected]>
Cr-Commit-Position: refs/heads/master@{#512456}
16 files changed