RemoteMacViews: Add AppShimHostBootstrap::GetAppShimPid

This will be queried in ExtensionAppShimHandler::OnProfileLoaded, where
we will check that the pid's signature.

Bug: 913362
Change-Id: I6fd144b26fad182d040f302f078a343cd2ab6cd9
Reviewed-on: https://chromium-review.googlesource.com/c/1373433
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615930}