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The BackForwardCache relies on pages to live in different
BrowsingInstance. It means the current page and every pages in the
BackForwardCache aren't sharing the same BrowsingInstance.

The problem is that normal history navigation can specify a SiteInstance
to reuse. It will be reused even when |force_browsing_instance_swap| is
true in the RenderFrameHostManagerImpl.

The basic idea of this CL: flush the BFCache before attempting a
cross-BrowsingInstance history navigation.

Bug: 993337, 999846
Change-Id: I26f1726a6ebe35eecb343b7310563e5001fc8f43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795902Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Lowell Manners <lowell@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695959}