Document: Use counters for stylesheetSet attributes

We add use counters for the following attributes:
attribute DOMString? selectedStylesheetSet;
readonly attribute DOMString? preferredStylesheetSet;

BUG=690600,690609

Change-Id: I0edf7fc6ec5e3e9295d36f18ad5853ffabfa6c1e
Reviewed-on: https://chromium-review.googlesource.com/745322
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bugs Nash <bugsnash@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512775}
7 files changed