[ios] Improve snapshotting

This CL updates snapshotting so that it does not
take snapshots if the snapshot will be stale.
We know it will be stale if the web state has
begun navigation.

Bug: 853569
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Icd5792c30ee08a562a55aca9520ef56a9702a306
Reviewed-on: https://chromium-review.googlesource.com/1147548
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578029}
5 files changed