media: Add static MojoCdmService::GetCdm().

This provides a way to statically lookup all CDMs created by MojoCdmService by
their CDM IDs.

BUG=545099

Review URL: https://codereview.chromium.org/1448303002

Cr-Commit-Position: refs/heads/master@{#360402}
4 files changed