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 7882fae2cf659c46d2e619b2023edfb85be2cd28..e157a5f67c1fa31230a468082ea47f0a94a16f23 100644
--- a/media/base/eme_constants.h
+++ b/media/base/eme_constants.h
@@ -90,8 +90,8 @@ 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,