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After a page enters the BackForwardCache, a user may navigate to a new
page on the same site as the cached page (See
BackForwardCacheBrowserTest.NavigateToTwoPagesOnSameSite).

In the above case, it's important not to reuse the RenderViewHost(s)
owned by the frames in the BackForwardCache, because this could lead to
two main frames (the one in the cache, and the one the user just
navigated to), existing at the same time for a single RenderViewHost
(not allowed).

To solve this, we can simply stop reusing RenderViewHosts that belong
to a page when it enters the BackForwardCache, and begin reusing them
again when it leaves the BackForwardCache.

This approach also allows us to remove a work-around in
navigation_controller_impl.cc that was aggressively deleting cached
frames.

Note: RenderViewHost is owned by RenderFrameHost, this CL changes
nothing WRT object lifetimes.

Change-Id: I18698fc4f737f3945ab41270bd07d1538ca096a5
Bug: 993337, 999846
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833616Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Lowell Manners <lowell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705043}