tickSharedTimer should process high prioirty tasks once.

In some cases the scheduler spends too much time running high priority tasks because Scheduler::tickSharedTimer calls swapQueuesAndRunPendingTasks twice.  This patch makes sure high prioirty tasks are only executed once per call.

BUG=391005,406217

Review URL: https://codereview.chromium.org/513123002

git-svn-id: svn://svn.chromium.org/blink/trunk@181059 bbb929c8-8fbe-4397-9dbb-9b2b20218538