[gin] Use variadic templates in gin/function_template.h

Replace pump.py generated gin/function_template.h with variadic template version.

This results in a concise code which does not require the use
of pump preprocessor.
Similar effort for base/callback.h is happening in https://codereview.chromium.org/610423003/

BUG=

Review URL: https://codereview.chromium.org/671433004

Cr-Commit-Position: refs/heads/master@{#301089}