[NearbyShare] Stub out delegate for system tray pod button

The Nearby Share pod button will need to make calls against code in
//chrome/browser, but it lives in //ash, which cannot depend on
//chrome. This delegate will provide the functionality needed for
the pod button.

Bug: 154866703
Test: unit_tests
Change-Id: Id0a04e56f48590e1770389ad2433fccedae105e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405640
Commit-Queue: Curt Clemens <cclem@google.com>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#807580}