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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}