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With ProactivelySwapBrowsingInstance, we will swap BrowsingInstances on
most cross-site navigations, which means there will be more
cross-process navigations on Android. This CL tries to reduce that by
trying to reuse the previous page's renderer process for the new page
when we do a proactive BrowsingInstance swap, if possible.

Note that the CL won't change cases where we are already forcing a
BrowsingInstance + process swap. Browser-initiated cross-site
navigations will cause BrowsingInstance + process swap no matter what,
and they will keep doing so even with this CL. Some renderer-initiated
cross-site navigations, however, do not trigger a BrowsingInstance swap
without us proactively swapping, so it is OK to reuse the process in
their case (because we would've done so anyways if we don't do a
proactive BrowsingInstance swap).

See https://docs.google.com/document/d/1b-uMmwzTlwGfpn8pjd59iA57RnBHVx4IOarWnxYYxQk/edit?usp=sharing
for more context (google-only)

Bug: 977562
Change-Id: I57d725aa1435de0ee54788de4bfe627161400725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2043521
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751515}