This flag will only render the first frame of each perf test,
regardless of their preferences for how many seconds to run.
It will be useful for speeding up the run time of the perf tests
on testing infrastructure that only cares about correctness.

BUG=chromium:725308
BUG=chromium:765321

Change-Id: I926f488c42f27ef23ef06a0159902613cff04080
Reviewed-on: https://chromium-review.googlesource.com/677306Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>