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.cc

@@ -107,4 +107,13 @@ const char kDisablePdfExtension[]         = "disable-pdf-extension";
 // upcoming version of Chromium.
 const char kEnableNPAPI[]                 = "enable-npapi";
 
+// Enable Widevine CDM.
+const char kEnableWidevineCdm[]           = "enable-widevine-cdm";
+
+// Path to Widevine CDM binaries.
+const char kWidevineCdmPath[]             = "widevine-cdm-path";
+
+// Widevine CDM version.
+const char kWidevineCdmVersion[]          = "widevine-cdm-version";
+
 }  // namespace switches