Fix setTimeoutCustom in worker thread

In worker thread, window doesn't exist so !window will raise a
referenceError. Check "typeof window" to fix this.

BUG tensorflow#6922

tfjs-core/src/platforms/platform_browser.ts

@@ -62,7 +62,8 @@ export class PlatformBrowser implements Platform {
   // Interleaving window.postMessage and setTimeout will trick the browser and
   // avoid the clamp.
   setTimeoutCustom(functionRef: Function, delay: number): void {
-    if (!window || !env().getBool('USE_SETTIMEOUTCUSTOM')) {
+    if (typeof window === 'undefined' ||
+        !env().getBool('USE_SETTIMEOUTCUSTOM')) {
       setTimeout(functionRef, delay);
       return;
     }