BackForwardCache: Additional recording at ShouldProactivelySwapBrowsingInstance

Before this CL, not-stored reasons are not recorded when the browsing
instance is not swapped. This means we are missing records in these
cases.

This CL fixes this by adding an additional record at
RenderFrameHostManager::ShouldProactivelySwapBrowsingInstance

Bug: 1134847
Change-Id: Id683b7f94de4e8cb81da0b5779f56a496d040b96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466002


Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824841}