Bug 1574122 - Provide a key for DOM Mutation breakpoint items r=jlast

Differential Revision: https://phabricator.services.mozilla.com/D42133

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

devtools/client/debugger/src/components/SecondaryPanes/DOMMutationBreakpoints.js

@@ -49,7 +49,7 @@ class DOMMutationBreakpointsContents extends Component<Props> {
       deleteBreakpoint,
     } = this.props;
     return (
-      <li>
+      <li key={breakpoint.id}>
         <input
           type="checkbox"
           checked={breakpoint.enabled}