WebGL compatibility: add stencil mask and ref restriction BUG=angleproject:1523 BUG=chromium:668223 Change-Id: I0726769c938fdfd50af0fad1cef1746d4af2a589 Reviewed-on: https://chromium-review.googlesource.com/422084 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>