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Causes the background task to be scheduled when limitless prefetching is enabled
and when the prefetch testing header value is changed. There is a five-second
delay built in when the background task is scheduled this way; this gives the
user a chance to change both limitless and the testing header before the task
starts.

Bug: 954252
Change-Id: Ib21e8e0fd4dca29226c2b3e2080da1afe7650ed8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574139Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Dan H <harringtond@chromium.org>
Commit-Queue: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654608}