Fix --use-xvfb argument to optimization_guide_gpu_unittests

The argument requires two dashes.

Change-Id: I3e0720d4e78f87d61d29138c34e8fbe1b5941a6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6669295
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#1478272}