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

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 a0861c4e5db41359f0e877c5ebce59008fde5c9d..61d37ea43b4362bfd37e8c6e96096820b7acdfaa 100644
--- a/media/base/cdm_context.h
+++ b/media/base/cdm_context.h
@@ -41,6 +41,26 @@ class MEDIA_EXPORT CdmContext {
   DISALLOW_COPY_AND_ASSIGN(CdmContext);
 };
 
+// An interface that provides the access to a CdmContext through |cdm_id|.
+// This is typically used when CDMs are hosted remotely.
+class MEDIA_EXPORT CdmContextProvider {
+ public:
+  virtual ~CdmContextProvider();
+
+  // Returns the CdmContext corresponding to |cdm_id|. Returns nullptr if no
+  // such CdmContext can be found.
+  // Note: |cdm_id| is irrelevant to GetCdmId() of the returned CdmContext.
+  // The former is used to locate the real CDM (the latter), which could just
+  // be a local CDM that doesn't support a CDM ID.
+  virtual CdmContext* GetCdmContext(int cdm_id) = 0;
+
+ protected:
+  CdmContextProvider();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(CdmContextProvider);
+};
+
 // 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;