media: Always define |cdm_id| in CdmContext.

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 2a3e8cd25ded665e6daf630152924b1f2b8a7d4d..b3e6a0b0ea0b270b9e436da9f5f22a2264ad5fd8 100644
--- a/media/base/cdm_context.h
+++ b/media/base/cdm_context.h
@@ -17,22 +17,22 @@ class Decryptor;
 // 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();