Add compiler warning flag -Wextra and fix related warnings.

Note: some of these warnings are enabled by a combination of -Wunused
(added earlier) and -Wextra.

Cherry-picked from aomedia/master: 4790a69

Change-Id: I322a1366bd4fd6c0dec9e758c2d5e88e003b1cbf

test/encode_test_driver.h

@@ -228,7 +228,6 @@ class EncoderTest {
 
   // Hook to be called to handle decode result. Return true to continue.
   virtual bool HandleDecodeResult(const aom_codec_err_t res_dec,
-                                  const VideoSource & /*video*/,
                                   Decoder *decoder) {
     EXPECT_EQ(AOM_CODEC_OK, res_dec) << decoder->DecodeError();
     return AOM_CODEC_OK == res_dec;