Bug 1620932 Fix React warning in devtools/client/responsive/components/Browser.js r=gl

Differential Revision: https://phabricator.services.mozilla.com/D66859

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

devtools/client/responsive/components/Browser.js

@@ -160,7 +160,7 @@ class Browser extends PureComponent {
     // access to browser features as regular browser tabs. The `swapFrameLoaders` platform
     // API we use compares such features before allowing the swap to proceed.
     return dom.iframe({
-      allowFullScreen: "true",
+      allowFullScreen: true,
       className: "browser",
       height: "100%",
       mozbrowser: "true",