media: Misc cleanup of CdmContext

- Clarify lifetime model.
- Clarify thread model.
- Remove GetClassIdentifier(). Moving foreard, we should add virtual
  methods to CdmContext to support platform-specific interfaces, instead
  of using static_cast.

TBR=jrummell@chromium.org,rkuroiwa@chromium.org

Bug: 821288
Test: No functionality change.
Change-Id: I13e30ed2c508855727219edd3975abf5d67e6721
Reviewed-on: https://chromium-review.googlesource.com/1014284Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551128}

media/base/cdm_context.cc

@@ -30,10 +30,6 @@ MediaCryptoContext* CdmContext::GetMediaCryptoContext() {
 }
 #endif
 
-void* CdmContext::GetClassIdentifier() const {
-  return nullptr;
-}
-
 void IgnoreCdmAttached(bool /* success */) {}
 
 }  // namespace media

media/base/cdm_context.h

@@ -20,6 +20,14 @@ class MediaCryptoContext;
 // An interface representing the context that a media player needs from a
 // content decryption module (CDM) to decrypt (and decode) encrypted buffers.
 // Typically this will be passed to the media player (e.g. using SetCdm()).
+//
+// Lifetime: The returned raw pointers are only guaranteed to be valid when the
+// CdmContext is alive, which is usually guaranteed by holding a CdmContextRef
+// (see below).
+//
+// Thread Model: Since this interface is used in many different contexts (e.g.
+// different processes or platforms), the thread model is not defined as part
+// of this interface. Subclasses must ensure thread safty.
 class MEDIA_EXPORT CdmContext {
  public:
   // Indicates an invalid CDM ID. See GetCdmId() for details.
@@ -29,34 +37,27 @@ class MEDIA_EXPORT CdmContext {
 
   // Gets the Decryptor object associated with the CDM. Returns nullptr if the
   // 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.
+  // occurs implicitly along with decoding).
   virtual Decryptor* GetDecryptor();
 
   // 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).
+  // TODO(crbug.com/804397): Use base::UnguessableToken for CDM ID.
   virtual int GetCdmId() const;
 
 #if BUILDFLAG(ENABLE_LIBRARY_CDMS)
-  // Returns a CdmProxyContext. The default implementation returns a nullptr
-  // to indidate that there is no context.
-  // If CdmProxy is not used, then this returns a nullptr.
-  // The pointer is owned by the callee.
+  // Returns a CdmProxyContext that can be used by hardware decoders/decryptors.
+  // Returns nullptr if CdmProxyContext is not supported, e.g. |this| is not
+  // hosted by a CdmProxy.
   virtual CdmProxyContext* GetCdmProxyContext();
 #endif  // BUILDFLAG(ENABLE_LIBRARY_CDMS)
 
 #if defined(OS_ANDROID)
   // Returns a MediaCryptoContext that can be used by MediaCodec based decoders.
-  // The returned object is only guaranteed to be valid during the CDM's
-  // lifetime.
   virtual MediaCryptoContext* GetMediaCryptoContext();
 #endif
 
-  // Returns a unique class identifier. Some subclasses override and use this
-  // method to provide safe down-casting to their type.
-  virtual void* GetClassIdentifier() const;
-
  protected:
   CdmContext();