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.cc

@@ -17,6 +17,7 @@
 #include "libcef/renderer/extensions/extensions_dispatcher_delegate.h"
 #include "libcef/renderer/extensions/extensions_renderer_client.h"
 #include "libcef/renderer/extensions/print_web_view_helper_delegate.h"
+#include "libcef/renderer/media/cef_key_systems.h"
 #include "libcef/renderer/pepper/pepper_helper.h"
 #include "libcef/renderer/render_frame_observer.h"
 #include "libcef/renderer/render_message_filter.h"
@@ -649,6 +650,11 @@ CefContentRendererClient::CreateBrowserPluginDelegate(
   }
 }
 
+void CefContentRendererClient::AddKeySystems(
+    std::vector<media::KeySystemInfo>* key_systems) {
+  AddCefKeySystems(key_systems);
+}
+
 void CefContentRendererClient::WillDestroyCurrentMessageLoop() {
   base::AutoLock lock_scope(single_process_cleanup_lock_);
   single_process_cleanup_complete_ = true;