media: Add Feature to control External Clear Key key system support

chrome/renderer/media/chrome_key_systems.cc

Index: chrome/renderer/media/chrome_key_systems.cc
diff --git a/chrome/renderer/media/chrome_key_systems.cc b/chrome/renderer/media/chrome_key_systems.cc
index 8169331c87d5c137255f8de55af70c61ea5b565e..1b2ebe671777638a8b4b98e7358f228deb2a2c3f 100644
--- a/chrome/renderer/media/chrome_key_systems.cc
+++ b/chrome/renderer/media/chrome_key_systems.cc
@@ -24,6 +24,11 @@
 #include "components/cdm/renderer/android_key_systems.h"
 #endif
 
+#if defined(ENABLE_PEPPER_CDMS)
+#include "base/feature_list.h"
+#include "media/base/media_switches.h"
+#endif
+
 #include "widevine_cdm_version.h" // In SHARED_INTERMEDIATE_DIR.
 
 // The following must be after widevine_cdm_version.h.
@@ -279,7 +284,8 @@ static void AddPepperBasedWidevine(
 void AddChromeKeySystems(
     std::vector<std::unique_ptr<KeySystemProperties>>* key_systems_properties) {
 #if defined(ENABLE_PEPPER_CDMS)
-  AddExternalClearKey(key_systems_properties);
+  if (base::FeatureList::IsEnabled(media::kExternalClearKeyForTesting))
+    AddExternalClearKey(key_systems_properties);
 
 #if defined(WIDEVINE_CDM_AVAILABLE)
   AddPepperBasedWidevine(key_systems_properties);