This CL does a number of things to cleanup MediaClient's
implementation and usage. This clears the way for an expanded role
in describing MediaCapabilities.

Remove CastMediaClient. Merge methods into CastContentRenderClient.
  ContentRenderClient is now the one-stop-shop for embedder
  customization. I initially  explored giving each embedder its own
  MediaClient, but this ended up splitting media customization
  between ContentRenderClient and MediaClient (some customization is
  too high level to move to MediaClient). The current route is also
  less code.

Remove all Chrome specific logic from RenderMediaClient.
  RenderMediaClient is now the plumbing for *all* content embedder
  customizations to be visible in media/. Chrome is just one of many
  embedders and much of the chrome specific code was stale anyway.

Remove KeySystemNameForUMA from MediaClient interface
  Decentralizing this logic was overly complicated.
  Now centralized to key_systems.cc.

Other little cleanups
  Centralize all media/ Rappor usage in MediaLog.
  Move cast key_systems_cast into chromecast/renderer/media.
  Simplify key_systems_unittest.cc

BUG=695264

Review-Url: https://codereview.chromium.org/2712983004
Cr-Commit-Position: refs/heads/master@{#463025}