GitLab

- Clarify lifetime model.
- Clarify thread model.
- Remove GetClassIdentifier(). Moving foreard, we should add virtual
  methods to CdmContext to support platform-specific interfaces, instead
  of using static_cast.

TBR=jrummell@chromium.org,rkuroiwa@chromium.org

Bug: 821288
Test: No functionality change.
Change-Id: I13e30ed2c508855727219edd3975abf5d67e6721
Reviewed-on: https://chromium-review.googlesource.com/1014284Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551128}