Work around a parameter-unused-but-set warning in gin/

GCC thinks that the create_flags parameter in the templated class
gin::Invoker constructor is going unused, even though it is definitely
being used as a part of a variadic template expansion. Convince GCC
that it is in fact being used by casting it to a void.

BUG=424334
TEST=Linux GN build of gin with is_clang=false
R=aa

Review URL: https://codereview.chromium.org/694063002

Cr-Commit-Position: refs/heads/master@{#302361}

gin/function_template.h

@@ -150,8 +150,13 @@ class Invoker<IndicesHolder<indices...>, ArgTypes...>
   // so it is guaranteed ArgumentHolders will be initialized (and thus, will
   // extract arguments from Arguments) in the right order.
   Invoker(Arguments* args, int create_flags)
-      : ArgumentHolder<indices, ArgTypes>(args, create_flags)...,
-        args_(args) {}
+      : ArgumentHolder<indices, ArgTypes>(args, create_flags)..., args_(args) {
+    // GCC thinks that create_flags is going unused, even though the
+    // expansion above clearly makes use of it. Per jyasskin@, casting
+    // to void is the commonly accepted way to convince the compiler
+    // that you're actually using a parameter/varible.
+    (void)create_flags;
+  }
 
   bool IsOK() {
     return And(ArgumentHolder<indices, ArgTypes>::ok...);