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Changes the behavior of instrumentation tests to automatically
restore any shared preference files modified by
--shared-prefs-file. Also adds a simple script for manually
applying a shared preference JSON file for cases where settings
that are normally only used during automated testing are
needed for manual use.

Change-Id: I94006c1a8e48d910ed2f7e0742f001d8d62b7b15
Reviewed-on: https://chromium-review.googlesource.com/1220557Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593218}