[unified-consent] Grey out sync section header when user is signed out

When the sync section expand button is disabled, the opacity of the
section header is reduced to make it look greyed out.

This means that the sync-section header is greyed out in the following
cases:
 - the user is signed out
 - the user is in a synced-paused state

Screenshots:
https://drive.google.com/file/d/1rDwDIqinKa2gl6QGnf2WoRkF2j28JOpH/view?usp=sharing
https://drive.google.com/file/d/1IFqP5M6pSI2bSwrTNphrQBGy1bO1Hce8/view?usp=sharing

Bug: 854273
Change-Id: I29e04b19f56c7c36a2c8e00f12929327b5612742
Reviewed-on: https://chromium-review.googlesource.com/1205836Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Thomas Tangl <tangltom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588923}

chrome/browser/resources/settings/people_page/sync_page.html

@@ -27,6 +27,10 @@
 <dom-module id="settings-sync-page">
   <template>
     <style include="settings-shared iron-flex">
+      #sync-section-toggle:not([actionable]) {
+        opacity: var(--cr-disabled-opacity);
+      }
+
       #create-password-box,
       #reset-sync-message-box-encryption,
       #reset-sync-message-box-user-events {