Add RenderWidgetProtector to simplify protecting a RenderWidget from destruction for the duration of a function.  Deploy it within RenderWidget.

Reviewed by Dan Bernstein.

* rendering/RenderWidget.cpp:
(WebCore::RenderWidget::setWidgetGeometry):
(WebCore::RenderWidget::updateWidgetPosition): Use a RefPtr rather than explicitly
ref'ing and deref'ing the node.
* rendering/RenderWidget.h:
* rendering/RenderWidgetProtector.h:
(WebCore::RenderWidgetProtector::RenderWidgetProtector):
(WebCore::RenderWidgetProtector::~RenderWidgetProtector):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@48358 268f45cc-cd09-0410-ab3c-d52691b4dbfc