Trace Tests: Refactor calibration logic.

Instead of calling glFinish every step we now run a few warmup loops
to try and figure out the right step interval. On each run we refine
the step interval until we reach a number that completes in the
specific time after a single glFinish call. This method will use the
specified test time much more consistenty.

Also a couple other improvements. Will use more test time on the bots
because previously they were using a calibration/warmup time of 1
second per run loop, and this bumps it up to 3 per run loop.

Long explanation below.

What we did before:

step()
glFinish()
step()
glFinish()
.. repeat for "gCalibrationTime" seconds
then take the number of steps, the actual time ran, and compute an
estimate for "mStepsToRun".

When we run for real, we go:

step()
step()
step()
.. repeat for "mStepsToRun" steps.
glFinish()

this has an obvious performance difference, because after each step()
(one frame in a trace) you had a complete stall.

The new code first does the above to compute a tentative mStepsToRun,
then uses the results of the second run to compute a much more accurate
mStepsToRun by using the "actual" time it took to run the estimate.

Bug: angleproject:6090
Change-Id: I0ff19873580e13f2af5a145a21f5b1c59fb15795
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3114606
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>