media: Always define |cdm_id| in CdmContext.

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
 // content decryption module (CDM) to decrypt (and decode) encrypted buffers.
 class MEDIA_EXPORT CdmContext {
  public:
-#if defined(ENABLE_BROWSER_CDMS)
+  // Indicates an invalid CDM ID. See GetCdmId() for details.
   static const int kInvalidCdmId = 0;
-#endif
 
   virtual ~CdmContext();
 
-  // Gets the Decryptor object associated with the CDM. Returns NULL if no
-  // Decryptor object is associated. The returned object is only guaranteed
-  // to be valid during the CDM's lifetime.
+  // Gets the Decryptor object associated with the CDM. Returns NULL if the CDM
+  // does not support a Decryptor. The returned object is only guaranteed to be
+  // valid during the CDM's lifetime.
   virtual Decryptor* GetDecryptor() = 0;
 
-#if defined(ENABLE_BROWSER_CDMS)
-  // Returns the CDM ID associated with |this|. May be kInvalidCdmId if no CDM
-  // ID is associated.
+  // Returns a unique ID associated with the CDM, which can be used by the
+  // recipient of this CdmContext to locate the real CDM. This is useful when
+  // the CDM is hosted remotely, e.g. in a different process.
+  // Returns kInvalidCdmId if the CDM does not support an ID, e.g. the CDM only
+  // supports a Decryptor interface.
   virtual int GetCdmId() const = 0;
-#endif
 
  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_