fix bug 41325. Make new entries in the keybinding for Mac so cmd+[ and cmd+] make forward and backward. r=saari

xpfe/browser/resources/content/mac/platformNavigationBindings.xul

@@ -7,6 +7,8 @@
     <!-- proper arrow key navigation, 4.xP -->
     <key id="goBackKb" xulkey="true" keycode="VK_LEFT" observes="Browser:Back"/>
     <key id="goForwardKb" xulkey="true" keycode="VK_RIGHT" observes="Browser:Forward"/>
+    <key id="goBackKbMac" xulkey="true" key="[" observes="Browser:Back"/>
+    <key id="goForwardKbMac" xulkey="true" key="]" observes="Browser:Forward"/>
     <key id="stopMac" xulkey="true" key="." onkeypres="BrowserStop();"/>
   </keyset>