Add gl_FragDepth support for D3D11/ES3. An incorrect check was blocking existing support we added for the extension. BUG=angleproject:1226 Change-Id: I335053b1bebb37fd9296b36702031bd34c1bb59e Reviewed-on: https://chromium-review.googlesource.com/313992 Tryjob-Request: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>