fix(scrolling): viewport ruler resize event running inside the NgZone (#12909)

Fixes the `resize` and `orientationchange` events from the `ViewportRuler` being run inside the `NgZone`, even though the intention is that they shouldn't be. The issue comes from the fact that we call `merge` outside the `NgZone`, however the thing that causes the event to be bound is the subscription right after it.

Fixes #12883.

src/cdk/scrolling/viewport-ruler.spec.ts

@@ -2,7 +2,7 @@ import {TestBed, inject, fakeAsync, tick} from '@angular/core/testing';
 import {ScrollingModule} from './public-api';
 import {ViewportRuler} from './viewport-ruler';
 import {dispatchFakeEvent} from '@angular/cdk/testing';
-
+import {NgZone} from '@angular/core';
 
 // For all tests, we assume the browser window is 1024x786 (outerWidth x outerHeight).
 // The karma config has been set to this for local tests, and it is the default size
@@ -140,5 +140,15 @@ describe('ViewportRuler', () => {
       expect(spy).toHaveBeenCalledTimes(1);
       subscription.unsubscribe();
     }));
+
+    it('should run the resize event outside the NgZone', () => {
+      const spy = jasmine.createSpy('viewport changed spy');
+      const subscription = ruler.change(0).subscribe(() => spy(NgZone.isInAngularZone()));
+
+      dispatchFakeEvent(window, 'resize');
+      expect(spy).toHaveBeenCalledWith(false);
+      subscription.unsubscribe();
+    });
+
   });
 });

src/cdk/scrolling/viewport-ruler.ts

@@ -36,11 +36,15 @@ export class ViewportRuler implements OnDestroy {
   private _invalidateCache: Subscription;
 
   constructor(private _platform: Platform, ngZone: NgZone) {
-    this._change = _platform.isBrowser ? ngZone.runOutsideAngular(() => {
-      return merge<Event>(fromEvent(window, 'resize'), fromEvent(window, 'orientationchange'));
-    }) : observableOf();
+    ngZone.runOutsideAngular(() => {
+      this._change = _platform.isBrowser ?
+          merge<Event>(fromEvent(window, 'resize'), fromEvent(window, 'orientationchange')) :
+          observableOf();
 
-    this._invalidateCache = this.change().subscribe(() => this._updateViewportSize());
+      // Note that we need to do the subscription inside `runOutsideAngular`
+      // since subscribing is what causes the event listener to be added.
+      this._invalidateCache = this.change().subscribe(() => this._updateViewportSize());
+    });
   }
 
   ngOnDestroy() {