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This breaks a dependency from AutomationManagerAura to the extensions
system. It should make AutomationManagerAura easier to test in unit
tests.

Bug: 910280
Test: browser_tests
Change-Id: I3e7036671d6e6140b6e19b72e7d6482e2fb6f376
Reviewed-on: https://chromium-review.googlesource.com/c/1357548Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613347}