Add improved performance marks for development mode (#3297)

app/javascript/mastodon/performance.js

@@ -0,0 +1,24 @@
+//
+// Tools for performance debugging, only enabled in development mode.
+// Open up Chrome Dev Tools, then Timeline, then User Timing to see output.
+// Also see config/webpack/loaders/mark.js for the webpack loader marks.
+//
+
+let marky;
+
+if (process.env.NODE_ENV === 'development') {
+  marky = require('marky');
+  require('react-addons-perf').start();
+}
+
+export function start(name) {
+  if (process.env.NODE_ENV === 'development') {
+    marky.mark(name);
+  }
+}
+
+export function stop(name) {
+  if (process.env.NODE_ENV === 'development') {
+    marky.stop(name);
+  }
+}