Create an interface for KeySystems, migrate WebEncryptedMediaClientImpl.

media/base/eme_constants.h

Index: media/base/eme_constants.h
diff --git a/media/base/eme_constants.h b/media/base/eme_constants.h
index b48309cef0e31a9aff8edd6df43596e409925837..c4511d2a61a8cd170e72ed45b532a518ed0d38c9 100644
--- a/media/base/eme_constants.h
+++ b/media/base/eme_constants.h
@@ -89,8 +89,8 @@ enum class EmeMediaType {
 };
 
 // Robustness values understood by KeySystems.
-// Note: KeySystems expects this ordering (in GetRobustnessConfigRule()),
-// changes may be required there if this list changes.
+// Note: key_systems.cc expects this ordering in GetRobustnessConfigRule(),
+// make sure to correct that code if this list changes.
 enum class EmeRobustness {
   INVALID,
   EMPTY,