Soft navigation popstate: Fix a typo and add clarifying dchecks

This CL fixes a typo, where some of the instances of
`soft_navigation_heuristics_task_id` were missing an "s", making it hard
to search for the whole flow.
It also expands an existing DCHECK, to make its cause clearer when it
is being hit in Canary.

Bug: 1403900
Change-Id: Ifdce1cc2882e647a9549c155f08e346e847c401f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4154992
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Yoav Weiss <yoavweiss@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1091354}