Instead of asking "do you have an entry not in this Vector", let's ask
"do you have an entry not in this HashTable".
(cherry picked from commit 861d46be3e69aeb1ca4a9e75029b4ce653bbcf47)
This allows us to use HeapFunction all of the way down, allowing us
to remove the Handle usage in after_session_callback for
create_new_child_navigable.
(cherry picked from commit fc83653f3c7a650229d2be3eef0f1738a9395db9)
Fixes a bug when session history traversal queue task could be
interrupted by another SHTQ task execution. For example:
1. SHTQ timer callback starts executing a task from the queue.
2. spin_until() is invoked during task execution.
3. SHTQ timer callback starts executing a task from the queue.
...because existance of this method conflicts with the purpose of
having a queue as it allows to start executing next task in the middle
of ongoing task.
For example:
1. SHTQ timer starts executing a task.
2. Task does SHTQ::process().
Another example:
1. SHTQ::process() start executing a task.
2. task does SHTQ::process().
This is not in the spec, but we need to make sure that "apply the
history step" for initial navigation to about:blank in iframe is
applied before subsequent navigations. Otherwise, "set ongoing
navigation" call during "about:blank" traversal might abort subsequent
ongoing navigation which is not expected to happen.
