Perform ANGLE_instanced_arrays validation for WebGL contexts. TEST=conformance2/rendering/instanced-arrays BUG=angleproject:1988 Change-Id: Ie513dcc7b1af540764fd5fe3790d3e6e3457f048 Reviewed-on: https://chromium-review.googlesource.com/475136 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>