Fix crash in IndexDataManagerPerfTest. 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/331768 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>