WebGLCompatibility: Allow GL_DEPTH_STENCIL_ATTACHMENT. Permits depth-stencil attachment points for WebGL. BUG=angleproject:2090 Change-Id: I7f5a7c63f2a4a76116ce5639833e5fd8d7f50ffb Reviewed-on: https://chromium-review.googlesource.com/764591 Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>