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Only prefetch the prefetch_url if the tab is in the foreground. Start a prefetch when a backgrounded tab is switched to the foreground. Only prefetch any URL once.

Bug: 976071
Change-Id: If0a34ef5203794d34993c90dbe8db370b1780b2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1667753
Commit-Queue: Sofiya Semenova <sofiyase@google.com>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671467}