Bug 1080205 - Part 1: Don't add clip rects to async-scrolled layers for root scrollframes, since the FrameMetrics to scroll those layers will be set on a ContainerLayer parent. r=tn, a=bajaj
Bug 1080205 - Part 2: Add test. r=tn, a=bajaj
Bug 1094083 - Make browser-element Handle nested alert gracefully. r=smaug, a=bajaj

Don't delete this._windowIDDict[outerID] until the last modal prompt is
closed.