Rewrite rtc::Bind using variadic templates.

I'd like to make a change to rtc::Bind in another CL, and that will
be easier if there are fewer lines of code to modify.

BUG=None

Review-Url: https://codereview.webrtc.org/2719683002
Cr-Original-Commit-Position: refs/heads/master@{#16838}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 08187d41ca45e4671e41856785590d2408c129b6
5 files changed