Add Widevine CDM support (issue #1631)

- Windows/Mac: Use `--enable-widevine-cdm` command-line flag to
  enable download of CDM binaries via the component updater.
- Linux: Use `--widevide-cdm-path` and `--widevine-cdm-version`
  command-line flags to load CDM binaries that already exist on
  the system.
- A cache-path value is usually required when CDM is enabled.

libcef/renderer/content_renderer_client.h

@@ -108,6 +108,7 @@ class CefContentRendererClient : public content::ContentRendererClient,
       content::RenderFrame* render_frame,
       const std::string& mime_type,
       const GURL& original_url) override;
+  void AddKeySystems(std::vector<media::KeySystemInfo>* key_systems) override;
 
   // MessageLoop::DestructionObserver implementation.
   void WillDestroyCurrentMessageLoop() override;