Remove gl::Context parameter from Observer functions. It was only used in exactly one instance in VertexArray. Instead we can cache a bool and avoid needing to pass it around. Will make signaling dirty easier in the Vulkan back-end. Bug: angleproject:3539 Change-Id: Ia570aec051a24a5280df49edc4345c54022b46ec Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1663838 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>