[aw] Add callback to proxy override methods

Add an optional callback to proxy override methods introduced in change
https://chromium-review.googlesource.com/c/chromium/src/+/1148384

This adds an optional callback developers can use to know when the
changes have been made to the network settings, since the methods are
not synchronous with the UI thread.

Bug: 851995
Change-Id: I08dc7cc87c57d213b67774d3391562ca07fd0217
Reviewed-on: https://chromium-review.googlesource.com/c/1258770
Commit-Queue: Laís Minchillo <laisminchillo@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599166}
11 files changed