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Add metric which record number of tabs after clean shutdown and
after the crash. If users have more tabs in metrics recorded
after the crash than there is a correlation between number of
tabs and crashiness. These metrics will help us understand if
reducing memory footprint from multiple tabs may have positive
impact on stability.

Bug: 1140990
Change-Id: I07026ac9671e72d9367faf01b5fa71d8d990bdc5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2516240
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: David Jean <djean@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824170}