Limitless prefetching preference followup

Fixes a few mistakes discovered after landing this change:

https://chromium-review.googlesource.com/c/chromium/src/+/1387960

Bug: 803584
Change-Id: I2ad01e8902c002813cc64f42cd451c1ebde38e3b
Reviewed-on: https://chromium-review.googlesource.com/c/1452773
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Commit-Queue: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629255}
2 files changed