MD Settings: add a11y secondary text (description) for Midi Devices in Content Settings

R=dschuyler@chromium.org
BUG=none
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2743733005
Cr-Commit-Position: refs/heads/master@{#456253}

chrome/browser/resources/settings/site_settings_page/site_settings_page.html

@@ -273,7 +273,8 @@
         </div>
       </div>
       <button class="subpage-arrow" is="paper-icon-button-light"
-          aria-label="$i18n{siteSettingsMidiDevices}"></button>
+          aria-label="$i18n{siteSettingsMidiDevices}"
+          aria-describedby="midiDevicesSecondary"></button>
     </div>
     <div class="settings-box" category$="[[ContentSettingsTypes.ZOOM_LEVELS]]"
         data-route="SITE_SETTINGS_ZOOM_LEVELS"