prefetch: Execute() will no longer call run func with null db

It's very easy to forget checking for !db, which explains crbug.com/860228.
Now Execute does the check for us. The caller must designate a value
to be passed to the result callback in the case of db failure.

BUG: 864592

Change-Id: Ie4e1ac0bcd60e9e0c8c30413af10d44ff0175b47
Reviewed-on: https://chromium-review.googlesource.com/1140277
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575709}
18 files changed