Also add a missing arrow from VertexArray to Context.

Bug: angleproject:3002
Change-Id: I2a30df0bb1c9681b4cf5178b75f1264ddf4ca2e2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592065Reviewed-by: Jamie Madill <jmadill@chromium.org>