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This CL adds the capability of doing proactive BrowsingInstance swap on
same-site main-frame navigations when the "SameSite" level is set on the
"ProactivelySwapBrowsingInstance" flag. With that, some main-frame
same-site navigations will result in a new BrowsingInstance,
SiteInstance, RenderFrameHost, RenderView, etc.

This CL does not include:
- Updates for tests that fails because they didn't expect a change of
RenderFrameHosts, etc. on same-site main frame navigations. These are
handled in other CLs.
- Fix to ensure order of unload handlers on same-site cross-RFH
navigations (will be fixed on a future CL)
- Fix to ensure WebPreferences are carried over for cross-RFH same-site
navigations (will be fixed on a future CL)

For more details, see doc: https://docs.google.com/document/d/1lHdkKLUe8H6ZP6ALwj-dsus7oYcuc93HkSCHCcerItg/edit?usp=sharing

Bug: 977562
Change-Id: I455e598d1c422984cb707bb3a8b54fa926bb5911
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2121522
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779712}