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The histogram shows that 99.9% of the time, less than
1000 BLOCK_SHUTDOWN tasks are posted during shutdown. This CL replaces
the histogram with a crash when the number is to high, which will be
more actionnable.

Note: Current histogram data shows that this crash will happen
less than 0.1% of the time.

Bug: 697731
Change-Id: I553caf93b0109a1bd44c5c841d0f557e6cad16cd
Reviewed-on: https://chromium-review.googlesource.com/c/1280670Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600892}