[bfcache] Allow bfcache to be used when DevTools are open

Do not prohibit pages from entering back-forward cache when DevTools
connection is present.

The existing logic handling RenderFrame change during the navigation
would work here as well, ensuring that the messages are always
dispatched to the active frame. Add a test to verify that.

Detailed explanation why this should work:
https://docs.google.com/document/d/1jFkfaKKLWzORKjnsICFsDrDaOBYZNMY07vApp4NCSgA/edit#

R=caseq@chromium.org,yangguo@chromium.org

Change-Id: Ib3f443e158bbd634b805b4eb27d0b34aa03cc094
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218275
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773667}