Drawing with a DYNAMIC usage array or element array buffer should
eventually lead to internally switching to a static usage, if the app
doesn't modify the data very often. This perf test simply renders a
bunch of random indexed triangles where both buffers are specified as
DYNAMIC. It should perform just as well as the static usage after a
warm-up period.

BUG=angleproject:1334

Change-Id: Ibe432d2122feaefc82d3c11cdf227f93ada82eda
Reviewed-on: https://chromium-review.googlesource.com/332578Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>