Adding text specifying that PerformanceMainFrameTiming.startTime should be identical to requestAnimationFrame callback value.

This change was discussed in GitHub Issue [Unclear about the startTime attribute on PerformanceMainFrameTiming object #9](https://github.com/w3c/frame-timing/issues/9#issuecomment-60422524). Wording changes was proposed by [natduca](https://github.com/natduca).