Do a browsing context swap when a contaminated prefetch is used.

This further reduces the visibility that these pages can have into each
others' activity. Fortunately, we do BCG swaps in several cases so this
isn't too complicated.

This is guarded by a FeatureParam so that we can separately control this
behavior in the event it is suspected of causing surprising effects. It
defaults to off but will default to on shortly if all goes well.

Bug: 1439246c
Change-Id: I80039754a10418708d575035368ad70f7f6deeba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5100677


Reviewed-by: Max Curran <curranmax@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1271723}