Change supported ES version to 3.1 on win.

Without GL_TEXTURE_2D_MULTISAMPLE enum, it will reach UNREACHABLE()
when running dEQP ES 3.1 tests on Win.

BUG=angleproject:1442

Change-Id: I55114854c4e9cba1caac10d3da5b416ef66a0d3e
Reviewed-on: https://chromium-review.googlesource.com/414606
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>