[bfcache] Don't cache pages that are still loading.

To avoid the complexity of caching and restoring still-loading pages,
for now simply don't cache pages that haven't finished loading.

Long term this logic will probably need to be extended to be more
complicated/permissive, to increase back-forward cache coverage.

Change-Id: Ic06d9b3d425d3df95b8d09bea1561c000a96acc4
Bug: 976697
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1692414
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Lowell Manners <lowell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680383}