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No-op cleanup extracted from https://codereview.chromium.org/2135413003/#ps40001

In this CL:

 - Explicitly document the threading requirements of the API after grokking its use of locks
   in some places and lack of in some others)
   - (It's mostly thread unsafe; except for a few calls which are okay from the owner
      as well as the underlying thread; and one method which is actually thread-safe)
   - Add assertions as such (and catch issues on try bots :-O -- http://crbug.com/629139)
   - Remove |thread_lock_| which is unnecessary under the documented conditions
     (delayed until http://crbug.com/629139 is fixed..).

 - C++11 member initializers (makes it cleaner to add new POD members and
                              makes constructors simpler -- e.g. |run_loop_| was uninitialized)
 - Assertions and tests for current API which allows Start/Stop/Start cycles
   but wasn't tested... (actually more than that is poorly/not tested in this class
   but I'm merely adding tests for parts I'm stressing in my actual follow-up CL).

 - Comment nits

 - Make event-based tests use WaitableEvent instead of sleeping + toggling a bool.

BUG=629716, 629139

Review-Url: https://codereview.chromium.org/2145463002
Cr-Commit-Position: refs/heads/master@{#407265}