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In the current profiler, if a single sample takes too long then the
sampling tasks get crowded and stops the main thread from running for
a long time. But usually the slowest part is the stack copying for
which the main thread is paused anyway. So, it is ok to take next sample
after the interval time after previous sample completes, instead of
strictly sampling every interval.

BUG=859260

Change-Id: Ib0671b1f465f5cf98caa01aeac615648b24a1cf4
Reviewed-on: https://chromium-review.googlesource.com/1234841
Commit-Queue: Siddhartha S <ssid@chromium.org>
Reviewed-by: Xi Cheng <chengx@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593375}