Bug 1535253 - Add back the .contentPane rule to passwordmgr.css that was removed accidentally. r=MattN

The .contentPane rule for /browser was consolidated and removed throughout the codebase but the .contentPane rule in passwordmgr.css is in /toolkit and shouldn't have been removed.

Differential Revision: https://phabricator.services.mozilla.com/D24922

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

toolkit/themes/shared/passwordmgr.css

@@ -4,6 +4,10 @@
 
 @namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 
+.contentPane {
+  margin: 9px 8px 5px 8px;
+}
+
 .actionButtons {
   margin: 0px 3px 6px 3px;
 }