https://bugs.webkit.org/show_bug.cgi?id=34943

Reviewed by Darin Fisher.

Currently, once a second LocalStorage takes all keys/values which have
been changed and sends them to a background thread to sync.  The problem
is that this background thread can get overwhelmed and stop being
responsive.  This means that if any other page tries to start using
LocalStorage (and thus initiates the initial import) that'll block on
all the previous syncs completing.

To mitigate this, I'm adding code so that we never schedule another
sync task when another is still running.  In order to keep the sync
tasks from growing exponentially when they do take longer than the
storage sync interval, I've also added a basic rate limiter.  No effort
is made to ensure fairness/ordering of what gets synced nor is there
any way for this rate to be changed because most normal uses of
LocalStorage really shouldn't be hitting these types of limits anyway.

The only behavioral change that's observible in JavaScript is time based
and thus it's not practical to make new tests that aren't racy.  The
existing layout tests cover LocalStorage pretty well, though.

* storage/StorageAreaSync.cpp:
(WebCore::StorageAreaSync::StorageAreaSync):
(WebCore::StorageAreaSync::scheduleFinalSync):
(WebCore::StorageAreaSync::syncTimerFired):
(WebCore::StorageAreaSync::performSync):
* storage/StorageAreaSync.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@55523 268f45cc-cd09-0410-ab3c-d52691b4dbfc