Adds a test to make sure encoder parms get to decoder.

This is meant as a framework for testing that encode parms make it
through to the decoder.

Change-Id: Idb86ee3668b45b4e73c23c6e4daef94b0650b786

test/decode_test_driver.h

@@ -97,6 +97,10 @@ class Decoder {
 
   bool IsVP8() const;
 
+  vpx_codec_ctx_t * GetDecoder() {
+    return &decoder_;
+  }
+
  protected:
   virtual vpx_codec_iface_t* CodecInterface() const = 0;