Mark bundle start time on web (#3147)

* Mark bundle start time on web

* TS

index.web.js

@@ -1,3 +1,5 @@
+import '#/platform/markBundleStartTime'
+
 import '#/platform/polyfills'
 import {registerRootComponent} from 'expo'
 import {doPolyfill} from '#/lib/api/api-polyfill'

src/platform/markBundleStartTime.web.ts

@@ -0,0 +1,2 @@
+// @ts-ignore Web-only. On RN, this is set by Metro.
+window.__BUNDLE_START_TIME__ = performance.now()