[BlackBerry] Checkerboard shown when clicking on error page buttons

[BlackBerry] Checkerboard shown when clicking on error page buttons
https://bugs.webkit.org/show_bug.cgi?id=90152
RIM PR #161867

Reviewed by George Staikos.

r121514 just made the race occur in lower possibility but can't avoid it.
This path is using another approach to fix the problem. When we are resetting
backingstore while committing a page we need to suspend backingstore and
screen update to make sure no other threads can update backingstore and screen.

* Api/BackingStore.cpp:
(BlackBerry::WebKit::BackingStorePrivate::resetTiles):
* Api/WebPage.cpp:
(BlackBerry::WebKit::WebPagePrivate::setLoadState):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121824 268f45cc-cd09-0410-ab3c-d52691b4dbfc