WebUI: Add common class to use on all the labels/text in settings row, update 'People' settings rows to use it

Bug: 686905
Change-Id: I97e750e55e73b54451f87e34f6b0570a6b89dd71
Reviewed-on: https://chromium-review.googlesource.com/c/1359331
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615799}
4 files changed