Mojo: Unify the disparate source and header files into 3

mojo/public/bindings/generators/cpp_templates/params_definition

+const uint32_t $METHOD_NAME = $METHOD_ID;
+class $CLASS {
+ public:
+  static $CLASS* New(mojo::Buffer* buf) {
+    return new (buf->Allocate(sizeof($CLASS)))
+        $CLASS();
+  }
+
+$SETTERS
+
+$GETTERS
+
+ private:
+  friend class mojo::internal::ObjectTraits<$CLASS>;
+
+  $CLASS() {
+    _header_.num_bytes = sizeof(*this);
+    _header_.num_fields = 3;
+  }
+
+  mojo::internal::StructHeader _header_;
+$FIELDS
+};
+MOJO_COMPILE_ASSERT(sizeof($CLASS) == $SIZE,
+                    bad_sizeof_${CLASS});