mash: Limit experimental ShelfWindowWatcher behavior to multi-process Mash

ShelfWindowWatcher proactively adds shelf items for certain windows.
Limit this experimental behavior to multi-process Mash.
Avoids unwanted shelf items for select file dialogs in SingleProcessMash.

Bug: 920714
Test: Select file dialogs do not get shelf items (except in multi-process Mash).
Change-Id: Ic5986a837d040f9bb8c59c203c6de87375e9091f
Reviewed-on: https://chromium-review.googlesource.com/c/1407081
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622087}
4 files changed