Track memory usage

Summary: ^

Reviewed By: passy

Differential Revision: D20035089

fbshipit-source-id: 5df97847e442c626af8445e0909ea5e07311cc7e

src/dispatcher/tracking.tsx

@@ -126,6 +126,7 @@ export default (store: Store, logger: Logger) => {
       app,
       sdkVersion,
       isForeground: state.application.windowIsFocused,
+      usedJSHeapSize: (window.performance as any).memory.usedJSHeapSize,
     };
 
     // reset dropped frames counter