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The trace event will be called "ThreadController active" and be in the
"base" tracing category. It will be activated for each active nested
loop ThreadController is involved in (i.e. all loops but purely native
nested loops -- undetectable).

The implementation uses a stack of active run-levels (replacing some
existing run-levels counters). This is is simpler than the alternatives
as the amount of conditions to enter/exit a run-level are too many to
hope to get manual accounting right:
Enter:
 - DoWork() coming from Run()
 - DoWork() coming from native HandleWorkMessage
 - BeforeDoInternalWork() coming from Run() or native as well
 - DoIdleWork coming from a native nested loop (this one is surprising
   but happens on Mac because the |idle_work_source_| gets invoked
   first when the |work_source_| isn't signaled and a native nested
   loop starts..!)
Exit:
 - DoIdleWork()
 - Quit() (was it already idle or not?)
 - Exit nested loop (was it already idle or not?)
 - Exit native task (was there a native nested loop within that native
   task that we need to "end" now?)

Two tricky cases:
  Nested native task : Native task => native nested loop => native task
 vs
  Subsequent native tasks: Native task A (returns), Native task B

BeforeDoInternalWork() was refactored as a scoped object because we
need to be able to observe when a native task is done to capture
Native Task => Nested native loop => End native task (needs to end
the native loop, we aren't otherwise notified about it). We need to
capture begin/end to be able to differentiate nested native tasks
from subsequent native tasks (e.g. in a nested RunLoop without
application tasks).

The stack-based approach + scoped native work handles all of these
cases across both ThreadController impls.

R=altimin@chromium.org, fdoray@chromium.org

      and regular runtime. Example trace @ goto.google.com/oasrn

Test: Locally with startup trace, nested loops (tab drag-out-of-window),
Bug: 899897, 1074019
Change-Id: I68831ce92a66da8b91a523d931846262182b5de5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2226216
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Oliver Li <olivierli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825082}