leilei@ pointed out that the latency of dialog rendering on Windows is highly unstable - varying by over a factor of 2 between runs.  This is not reflected on Mac or Linux.

https://chromeperf.appspot.com/report?sid=67809cc0a8535b3f12bfe305464db8fc41909160cc0f6fe0f7b5f4aa8f5c36f0

It would be helpful to understand what is Windows-specific about the rendering path and whether we can do anything to address it.   This issue may also be affecting the stability of the HTML5 video mirroring test ( Bug 678315 ).