Issue #26 - Part 2b: Stub out GMPDecryptorProxy functions.

If EME isn't built, these functions would have no use and the callback
structures are unavailable. Stubbing these out will still allow the
callers to work unchanged, but in effect simply always return without
dispatching a promised task.
This fixes the dependency issue from part 2a.

dom/media/gmp/GMPDecryptorProxy.h

@@ -6,7 +6,9 @@
 #ifndef GMPDecryptorProxy_h_
 #define GMPDecryptorProxy_h_
 
+#ifdef MOZ_EME
 #include "mozilla/DecryptorProxyCallback.h"
+#endif
 #include "GMPCallbackBase.h"
 #include "gmp-decryption.h"
 #include "nsString.h"
@@ -15,8 +17,13 @@ namespace mozilla {
 class CryptoSample;
 } // namespace mozilla
 
+#ifdef MOZ_EME
 class GMPDecryptorProxyCallback : public DecryptorProxyCallback,
                                   public GMPCallbackBase {
+#else
+class GMPDecryptorProxyCallback : public GMPCallbackBase {
+#endif
+
 public:
   virtual ~GMPDecryptorProxyCallback() {}
 };