MD-Settings: A11y - Label power source and stylus drop downs.

R=derat@chromium.org
BUG=710691
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2817973002
Cr-Commit-Position: refs/heads/master@{#464552}