GitLab

Updates RFHM::CommitPendingIfNecessary to actually commit in a few more
cases and not crash when navigations happen simultaneously.

Also updates RFHM::CommitPending to make some logic clearer.

BUG=376094

Review URL: https://codereview.chromium.org/1036243003

Cr-Commit-Position: refs/heads/master@{#324234}