VertexArray: on destroy, remove observer on element array buffer Otherwise there will be a null pointer dereference in gl::Buffer::onContentsChange(). Signed-off-by: Steven Noonan <steven@valvesoftware.com> Bug: angleproject:6599 Change-Id: I73388b6d0ccc357ca1944452a18ccf462dd8046c Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3236206 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>