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/recursive_parser_test.cc

@@ -32,12 +32,12 @@ class FailParser : public ElementParser {
  public:
   FailParser() { EXPECT_FALSE(true); }
 
-  Status Init(const ElementMetadata& metadata,
-              std::uint64_t max_size) override {
+  Status Init(const ElementMetadata& /* metadata */,
+              std::uint64_t /* max_size */) override {
     return Status(Status::kInvalidElementSize);
   }
 
-  Status Feed(Callback* callback, Reader* reader,
+  Status Feed(Callback* /* callback */, Reader* /* reader */,
               std::uint64_t* num_bytes_read) override {
     *num_bytes_read = 0;
     return Status(Status::kInvalidElementSize);