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This change:
  - Does a separate pass on all histograms, validating them, before
    starting to prepare their deltas. This will confirm that data is
    bad already before we start processing them (i.e. verify that it's
    not the processing code having some side effect).
  - Counts the number of corrupted histograms to tell us if it's a single
    bad value or more widespread corruption.
  - Keeps track of the last histogram that someone logged something to,
    so that we could have a trail of the last thing that happened before
    corruption.
  - Moves a previously added instrumentation call to within the correct if
    block.

BUG=736675,744734

Change-Id: I4b860ad6e977c1555409bd628f9e4e5147e61654
Reviewed-on: https://chromium-review.googlesource.com/581418
Commit-Queue: Alexei Svitkine (slow) <asvitkine@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488815}