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This is part of the migration phase for TaskScheduler, design doc:
https://docs.google.com/document/d/1S2AAeoo1xa_vsLbDYBsDHCqhrkfiMgoIPlyRi6kxa5k/edit

Also generalizes BrowserThreadImpl away from the underlying thread's MessageLoop
and id in favor of TaskRunners and BrowserThreadState.

Cool side-effect: BrowserThreadImpl::StartWithOptions() no longer blocks on the
underlying thread starting (it used to via GetThreadId()), it now takes a reference
to its TaskRunner and lets the thread itself start on its own schedule :).
This wasn't previously intended to block (BrowserThreadImpl::StartAndWaitForTesting() is
supposed to be used for that) but had unintentionally regressed in http://crrev.com/408295
because GetThreadId() implicitly waits (http://crbug.com/672977).

Another nice improvement is that ~BrowserThreadImpl() now cleans the BrowserThreadGlobals
associated with it, this has the side-effect to force proper destruction order of
TestBrowserThread in tests which brought forth a few precursor cleanups
(https://crbug.com/653916#c24) which will from now on be enforced by this CL.

When redirection is disabled, the logic should be the exact same as before.
 - Threads are brought up.
 - Tasks are posted to their MessageLoop (albeit through their TaskRunner now).
 - On shutdown, threads are joined one by one.

When redirection is enabled, we try to mimic this logic.
 - Redirection TaskRunners are only live (|accepting_tasks|) for the same period that the
   matching thread would be.
 - On shutdown, we block on each TaskRunner's queue being flushed one-by-one
   in the same order as in the no-redirection case (this almost identical to
   real threads join % one slight difference documented in detail in
   BrowserThreadImpl::StopRedirectionOfThreadID()).

BUG=653916, 672977
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_tsan_rel_ng

TEST=
 A) "out\Release\chrome.exe"
    Runs/shuts down the exact same as before.

 B) "out\Release\chrome.exe --force-fieldtrials=BrowserScheduler/Enabled/ --force-fieldtrial-params=BrowserScheduler.Enabled:Background/3;3;1;0;10000/BackgroundFileIO/3;3;1;0;10000/Foreground/3;3;1;0;10000/ForegroundFileIO/3;3;1;0;10000/RedirectSequencedWorkerPools/true/RedirectNonUINonIOBrowserThreads/true"
    Runs/shuts down smoothly and chrome://tracing confirms that redirected BrowserThreads are running on TaskSchedulerWorkers.

Review-Url: https://codereview.chromium.org/2464233002
Cr-Commit-Position: refs/heads/master@{#439139}