On desktop, changing tabs away and back when a permission prompt is
active causes an additional Permissions.Prompt.Shown count to be logged.
This is somewhat unintuitive, so this patch removes these events from
the histogram. I estimate this accounts for 1-3% error (depending on
permission type), which should be small enough that we needn't record
the count of re-shows.

Change-Id: I397f41d5c576fe7d2e4c2f33c346ffcc759eecd1
Reviewed-on: https://chromium-review.googlesource.com/717996


Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508974}