Migrate PepperBrowserConnection to use OnceCallback.

Bug: 714018

Change-Id: I780e94380f65214921eb87e9ecb4fd8fa33fe721
Reviewed-on: https://chromium-review.googlesource.com/c/1357506
Reviewed-by: Raymes Khoury <raymes@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613803}
8 files changed