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.
