Implement Frame Timing API iframe Beacon

[jdreetz]

Not sure if this is the right place to leave this, but it appears that a cross browser version of mozPaintCount is being standardized:

https://w3c.github.io/frame-timing/ https://code.google.com/p/chromium/issues/detail?id=65348 https://code.google.com/p/chromium/issues/detail?id=444122

If this works in a similar fashion to mozPaintCount, it could be a viable non-Flash measurement method. According to the last link above, Safari, IE, and FF will be supporting it, although they do not describe how they've verified that assertion.