SingletonFor(type, state) returns NULL and does nothing whenever SingletonFor(type) also return NULL. So checking this beforehand is redundant.

This is my first CL, so I also use it to get familiar with the system.

R=titzer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1862753003

Cr-Commit-Position: refs/heads/master@{#35285}