Don't clear window.name on same-site cross-BrowsingInstance navigations.

The current code clears window.name on all main frame navigations that
swap BrowsingInstance (BrowsingContextGroup). But we now proactively
swap BrowsingInstance on same-site navigations too, where we should not
clear window.name based on the html spec:
https://html.spec.whatwg.org/multipage/browsing-the-web.html#resetBCName

This is a retry for the CL reverted in
https://chromium-review.googlesource.com/c/chromium/src/+/2797115, in
which the render-initiated navigation in the first test case added could
result in BrowsingInstances swap with BFCache enabled, a condition check
is added in this CL for that.

Bug: 1090128
Change-Id: I6954588538b5c0ad727921f565c8a84bfae54d9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2800793
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Shuran Huang <shuuran@chromium.org>
Cr-Commit-Position: refs/heads/master@{#868967}