webm_parser: Enable usage of werror.

Mass warning clean up. Mainly:
- Explicit casts of numeric literals to avoid signed/unsigned compare
  warnings.
- Commenting out of unused function arg names.

Change-Id: I0e70393a5743ae984035d43712c724d4ccd12f9d

webm_parser/tests/date_parser_test.cc

@@ -49,7 +49,7 @@ TEST_F(DateParserTest, IncrementalParse) {
 
   IncrementalParseAndVerify();
 
-  EXPECT_EQ(0xFEDCBA9876543210, parser_.value());
+  EXPECT_EQ(static_cast<std::int64_t>(0xFEDCBA9876543210), parser_.value());
 }
 
 }  // namespace