media: Add MediaDrmBridgeCdmContext.

media/base/cdm_context.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_BASE_CDM_CONTEXT_H_
 #define MEDIA_BASE_CDM_CONTEXT_H_
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "media/base/media_export.h"
 
 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.
   static const int kInvalidCdmId = 0;
 
   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:
   CdmContext();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CdmContext);
 };
 
 // Callback to notify that the CdmContext has been completely attached to
 // the media pipeline. Parameter indicates whether the operation succeeded.
 typedef base::Callback<void(bool)> CdmAttachedCB;
 
 // A dummy implementation of CdmAttachedCB.
 MEDIA_EXPORT void IgnoreCdmAttached(bool success);
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_CDM_CONTEXT_H_