Capture periodic screenshots relative to first paint

[GoogleCodeExporter]

Periodic screenshots are captured relative to the run start, not the first 
paint callback, so sometimes we miss the first paint screen.  This is 
especially bad if our page loads quickly (<2 seconds) but our screenshots are 
infrequent (e.g. once per second).

This also causes some unrelated bugs, e.g. screenshot before first paint, a 
long delay until the first screenshot, etc.

Instead of capturing periodic screenshots relative to the run start time, the 
iOS BZAgent should capture a screenshot on the first paint callback and then 
periodically relative to the first paint time.

The agent should still capture the usual marker screens as before:
  1) on "startSession", to capture the initial "blank" screen
  2) on "startRender" (same as first paint callback?)
  3) on "docComplete"

Let's fix iOS first, then the other agents (Android/etc).

Original issue reported on code.google.com by z...@google.com on 26 Nov 2012 at 4:32