It should be equivalent to GL_MAX_VARYING_COMPONENTS for ES2.

BUG=angleproject:1038

Change-Id: Ic2fe6c27a0ebd6259d70f8d28edb7d4bdd5d0c35
Reviewed-on: https://chromium-review.googlesource.com/286526Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>