media: Add MediaDrmBridgeCdmContext.

media/base/cdm_context.h

Index: media/base/cdm_context.h
diff --git a/media/base/cdm_context.h b/media/base/cdm_context.h
index 82e088d1200813a25e5008432461442db54324e0..d41281ec53dcfef71a4334da3bcd909da3ced13c 100644
--- a/media/base/cdm_context.h
+++ b/media/base/cdm_context.h
@@ -13,9 +13,9 @@ namespace media {
 
 class Decryptor;
 
-// An interface representing the context that a media pipeline needs from a
+// An interface representing the context that a media player needs from a
 // content decryption module (CDM) to decrypt (and decode) encrypted buffers.
-// Only used for implementing SetCdm().
+// This is used to pass the CDM to the media player (e.g. SetCdm()).
 class MEDIA_EXPORT CdmContext {
  public:
   // Indicates an invalid CDM ID. See GetCdmId() for details.
@@ -24,15 +24,14 @@ class MEDIA_EXPORT CdmContext {
   virtual ~CdmContext();
 
   // Gets the Decryptor object associated with the CDM. Returns nullptr if the
-  // CDM does not support a Decryptor. Must not return nullptr if GetCdmId()
-  // returns kInvalidCdmId. The returned object is only guaranteed to be valid
-  // during the CDM's lifetime.
+  // CDM does not support a Decryptor (i.e. platform-based CDMs where decryption
+  // occurs implicitly along with decoding). The returned object is only
+  // guaranteed to be valid during the CDM's lifetime.
   virtual Decryptor* GetDecryptor() = 0;
 
-  // Returns an ID that identifies a CDM, or kInvalidCdmId. The interpretation
-  // is implementation-specific; current implementations use the ID to locate a
-  // remote CDM in a different process. The return value will not be
-  // kInvalidCdmId if GetDecryptor() returns nullptr.
+  // Returns an ID that can be used to find a remote CDM, in which case this CDM
+  // serves as a proxy to the remote one. Returns kInvalidCdmId when remote CDM
+  // is not supported (e.g. this CDM is a local CDM).
   virtual int GetCdmId() const = 0;
 
  protected: