[bg tab loading] Change when has_restored_tabs_to_load_ is updated.

has_restored_tabs_to_load_ is updated by UpdateHasRestoredTabsToLoad()
at key moments when whether there are restored tabs to load may have
changed. It is important not to call UpdateHasRestoredTabsToLoad() in
the middle of a state transition (e.g. when a page has been removed
from page_nodes_to_load_ but not added to page_nodes_loading_),
otherwise the wrong state may be calculated. This CL changes
when UpdateHasRestoredTabsToLoad() is called to ensure that it is
not called in the middle of a state transition, which previously caused
a CHECK failure (see test added in this CL).

Bug: 437957111
Change-Id: Ia022f616e316650655be47b63a3851a9ff813830
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6839289
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Alison Gale <agale@chromium.org>
Commit-Queue: Jian Lin <jianlin1@Microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1504785}