media: Support creation and SetCdm() for mojo based CDM.

media/mojo/interfaces/content_decryption_module.mojom

Index: media/mojo/interfaces/content_decryption_module.mojom
diff --git a/media/mojo/interfaces/content_decryption_module.mojom b/media/mojo/interfaces/content_decryption_module.mojom
index 24d407236a585109d54aaebdebd26b4eefd6327e..3f4019674fa50f816bc8f766e8311c6cfd6c51e1 100644
--- a/media/mojo/interfaces/content_decryption_module.mojom
+++ b/media/mojo/interfaces/content_decryption_module.mojom
@@ -38,7 +38,7 @@ struct CdmPromiseResult {
   bool success;
   CdmException exception;
   uint32 system_code;
-  string error_message;
+  string? error_message;
 };
 
 // Transport layer of media::CdmKeyInformation (see
@@ -111,18 +111,10 @@ interface ContentDecryptionModule {
   // |session_id|.
   RemoveSession(string session_id) => (CdmPromiseResult result);
 
-  // Assigns the |cdm_id| to the CDM, and retrieves the |decryptor| associated
-  // with this CDM instance.
-  // A CDM implementation must choose to support either an explicit or implicit
-  // decryptor:
-  // - Explicit (non-null) decryptor: The client (e.g. media pipeline) will use
-  //   the |decryptor| directly to decrypt (and decode) encrypted buffers.
-  // - Implicit (null) decryptor: The client (e.g. media pipeline) will use the
-  //   |cdm_id| to locate a decryptor and associate it with the client.
-  // Note: In Chromium GetCdmContext() is a sync call. But we don't have an easy
-  // way to support sync calls on a mojo interface. Instead the client should
-  // generate a client side ID and pass it to the service.
-  GetCdmContext(int32 cdm_id, Decryptor&? decryptor);
+  // Retrieves the |decryptor| associated with this CDM instance.
+  // If the returned |decryptor| is not null, it will be used by the client
+  // (e.g. media pipeline) directly to decrypt (and decode) encrypted buffers.
+  GetDecryptor(Decryptor&? decryptor);
 };
 
 // Session callbacks. See media/base/media_keys.h for details.