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With this CL, PerformanceManager(Impl)::CallOnGraph can safely be
called from any sequence, at any time. The callback does not run if
the call is made before creation of after destruction of the
Performance Manager.

Implementation details: CallOnGraph always posts a task to the
Performance Manager sequence. When the task runs on the
Performance Manager sequence, it checks if a global pointer to
the Performance Manager if set. This is not racy because the
pointer is only modified from the Performance Manager sequence.
If the pointer is set, the task gets the Graph from it and invokes
the callback that was passed to CallOnGraph. Otherwise, the
callback does not run.

Bug: 980533
Change-Id: I3d640e0ae13d012d76bab1483a5d59afb977c1a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1849213
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704602}