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We now always abort invalid loads. According to UMA, they don't happen
anymore, so this should only affect future bugs.

Also, move the UMA reporting to the right thread.

BUG=none
R=mmenke@chromium.org,mef@chromium.org,nasko@chromium.org

Review URL: https://codereview.chromium.org/572273002

Cr-Commit-Position: refs/heads/master@{#295107}