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/browser/content_browser_client.cc

@@ -722,6 +722,8 @@ void CefContentBrowserClient::AppendExtraCommandLineSwitches(
       switches::kPpapiFlashPath,
       switches::kPpapiFlashVersion,
       switches::kUncaughtExceptionStackSize,
+      switches::kWidevineCdmPath,
+      switches::kWidevineCdmVersion,
     };
     command_line->CopySwitchesFrom(*browser_cmd, kSwitchNames,
                                    arraysize(kSwitchNames));
@@ -734,6 +736,8 @@ void CefContentBrowserClient::AppendExtraCommandLineSwitches(
     static const char* const kSwitchNames[] = {
       switches::kPpapiFlashPath,
       switches::kPpapiFlashVersion,
+      switches::kWidevineCdmPath,
+      switches::kWidevineCdmVersion,
     };
     command_line->CopySwitchesFrom(*browser_cmd, kSwitchNames,
                                    arraysize(kSwitchNames));