Bug 1572641 Part 4: Give the More Info button a label for screen readers, r=johannh

Differential Revision: https://phabricator.services.mozilla.com/D41352

--HG--
extra : moz-landing-system : lando

browser/components/controlcenter/content/protectionsPanel.inc.xul

@@ -24,7 +24,8 @@
                role="heading" aria-level="1">
           <html:span id="protections-popup-mainView-panel-header-span"/>
         </label>
-        <toolbarbutton id="protections-popup-info-button" class="panel-info-button">
+        <toolbarbutton id="protections-popup-info-button" class="panel-info-button"
+                       aria-label="&protections.etpMoreInfo.label;">
           <image/>
         </toolbarbutton>
         <description id="protections-popup-toast-panel-tp-on-desc">&protections.etpON.header;</description>

browser/locales/en-US/chrome/browser/browser.dtd

@@ -741,6 +741,9 @@ you can use these alternative items. Otherwise, their values should be empty.  -
 
 <!ENTITY protections.etpON.header "Enhanced Tracking Protection is ON for this site">
 <!ENTITY protections.etpOFF.header "Enhanced Tracking Protection is OFF for this site">
+<!-- LOCALIZATION NOTE:
+     protections.etpMoreInfo.label: The text a screen reader speaks when focused on the info button. -->
+<!ENTITY protections.etpMoreInfo.label "More information about Enhanced Tracking Protection">
 <!ENTITY protections.settings.label "Protection Settings">
 <!ENTITY protections.report.label "Show Full Report">
 <!-- LOCALIZATION NOTE: