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When the tab is brought to the foreground, preconnect to the
origin of the main frame request. This preconnection is done
at most once, and is helpful in case user clicks on a link.

Change-Id: I6786c14ad7be0921bb309aabac5a75cff9819db8
Bug: 908725
Reviewed-on: https://chromium-review.googlesource.com/c/1350209Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611107}