Using a mock BufferFactory would sometimes leave us with a nullptr in
the VertexBufferInterface class. This would lead to a null deref in
the perf tests only. Fix this by adding a null check.

BUG=593541

Change-Id: I98685a9990124f521cd26ed81a26aeb11aafaf0d
Reviewed-on: https://chromium-review.googlesource.com/331768Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>