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/common/cef_switches.h

@@ -45,6 +45,9 @@ extern const char kEnableSystemFlash[];
 extern const char kDisableScrollBounce[];
 extern const char kDisablePdfExtension[];
 extern const char kEnableNPAPI[];
+extern const char kEnableWidevineCdm[];
+extern const char kWidevineCdmPath[];
+extern const char kWidevineCdmVersion[];
 
 }  // namespace switches