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/virtual_block_parser_test.cc

@@ -52,7 +52,7 @@ TEST_F(VirtualBlockParserTest, ValidBlock) {
 
   const VirtualBlock virtual_block = parser_.value();
 
-  EXPECT_EQ(1, virtual_block.track_number);
+  EXPECT_EQ(static_cast<std::uint64_t>(1), virtual_block.track_number);
   EXPECT_EQ(0x1234, virtual_block.timecode);
 }
 
@@ -67,7 +67,7 @@ TEST_F(VirtualBlockParserTest, IncrementalParse) {
 
   const VirtualBlock virtual_block = parser_.value();
 
-  EXPECT_EQ(2, virtual_block.track_number);
+  EXPECT_EQ(static_cast<std::uint64_t>(2), virtual_block.track_number);
   EXPECT_EQ(-2, virtual_block.timecode);
 }