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/master_parser_test.cc

@@ -216,7 +216,7 @@ TEST_F(MasterParserTest, UnknownSize) {
 
   ParseAndVerify(kUnknownElementSize);
 
-  EXPECT_EQ(5, reader_.Position());
+  EXPECT_EQ(static_cast<std::uint64_t>(5), reader_.Position());
 }
 
 // Consecutive elements with unknown size should parse without issues, despite