Use RefPtr in PageWidgetDelegate and guard RenderView

Layout and recalcStyle can both execute script (unfortunately) and can
also call into plugins. We should RefPtr the FrameView in the
PageWidgetDelegate to make sure that nothing can blow away the FrameView
while we're in the middle of updating it.

I also protected the RenderView. If the Frame was detached by the script
then the RenderView would be null and we'd crash here.

It's not clear how to write a test for this, you'd need to blow away the top
level frame from inside recalcStyle or layout. 

R=abarth@chromium.org,ojan@chromium.org,inferno@chromium.org

Review URL: https://codereview.chromium.org/149983003

git-svn-id: svn://svn.chromium.org/blink/trunk@166181 bbb929c8-8fbe-4397-9dbb-9b2b20218538