Revert 144488 - For unit tests, track additions to AtExitManager and warn.

Revert 144488 - For unit tests, track additions to AtExitManager and warn.

While trying to bullet proof a unit test, I had trouble getting very far when
running all tests in shuffle mode. Tracked that back to a few other tests doing
stuff that accessed Singleton()s outside of a test-scoped
ShadowingAtExitManager. Seemed to me that should be an invariant around any
unit test, so created this towards that end, hopefully helping stabilize out
unit_tests a bit more.

BUG=133403
Review URL: https://chromiumcodereview.appspot.com/10582012

TBR=scottbyer@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=148405