Added Reset method to TwopassStatsStore

The stats buffer needs to be reset between runs of the
encoder. I added a Reset() function to TwopassStatsStore
and called it at the beginning of each encode.

This enables us to run multiple encodes which was
previously not possible since there was no way to reset
the stats between runs.

Change-Id: Iebb18dab83ba9331f009f764cc858609738a27f9
diff --git a/test/encode_test_driver.h b/test/encode_test_driver.h
index 3d74b74..f065492 100644
--- a/test/encode_test_driver.h
+++ b/test/encode_test_driver.h
@@ -65,6 +65,10 @@
     return buf;
   }
 
+  void Reset() {
+    buffer_.clear();
+  }
+
  protected:
   std::string  buffer_;
 };