Fix UpdateAttribBindingByVertexAttribDivisor test. Test should clear before drawing. With the clears added it became apparent the second draw wasn't working because an attrib was left in instanced mode after the first draw. BUG=angleproject:2672 Change-Id: I6c3ee7302d24babb80a9b9affcf357145bb03dcb Reviewed-on: https://chromium-review.googlesource.com/c/1306822 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>