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In some situations, Start() would be called from both the constructor
and the observer notification method quickly, yielding to multiple
concurrent flows. Prevent those.

In addition, redo the tests so that they test both work from the
observer notification method and from an existing policy. Also add
a test that triggers the concurrency situation and verifies that
there is only one flow running in that situation.

BUG=chromium:887683
TEST=unit_tests --gtest_filter=*EnrollmentPolicyObserverTest*
CQ-DEPEND=CL:1234373

Change-Id: Ice6b46e19bf77a837b4c4908f332e9af6c5189f5
Reviewed-on: https://chromium-review.googlesource.com/1237242
Commit-Queue: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594751}