The Clear command in the draw call benchmark was not being properly
no-op'ed. This would cause the driver to get a huge queue of GL
command, which it would only flush when the test was torn down. This
manifested as a 5+ second stall on test teardown.

Looking at the clear command, it does some work with binding
and then calling GL functions, so properly filtering out the driver
commands would be a bit more complicated. For now, we can hack in a
workaround for the draw call test (the only one that uses the GL
NULL renderer at the moment) to not call clear when it would give
us problems.

BUG=620687

Change-Id: I64a2ed1883e8d3bf0d4c423c8bce0902ae9bf8df
Reviewed-on: https://chromium-review.googlesource.com/352828Reviewed-by: Geoff Lang <geofflang@chromium.org>