Change Ice204e86 identified a problem with bitrate undershoot due to
low precision in the timestamps passed to the library. This patch
takes a different approach by calculating the duration of this frame
and passing it to the library, rather than using a fixed duration
and letting the library average it out with higher precision
timestamps. This part of the fix only applies to vpxenc.

This patch also attempts to fix the problem for generic applications
that may have made the same mistake vpxenc did. Instead of
calculating this frame's duration by the difference of this frame's
and the last frame's start time, we use the end times instead. This
allows the framerate calculation to scavenge "unclaimed" time from
the last frame. For instance:

  start |  end  | calculated duration
  ======+=======+====================
    0ms    33ms   33ms
   33ms    66ms   33ms
   66ms    99ms   33ms
  100ms   133ms   34ms

Change-Id: I92be4b3518e0bd530e97f90e69e75330a4c413fc