fix(live-announcer): avoid triggering a reflow when reading directive content

Uses `textContent`, instead of `innerText`, to avoid triggering a reflow when reading off the content of the directive element.

Author: crisbeto

src/cdk/a11y/live-announcer/live-announcer.ts

@@ -117,13 +117,16 @@ export class CdkAriaLive implements OnDestroy {
         this._subscription.unsubscribe();
         this._subscription = null;
       }
-    } else {
-      if (!this._subscription) {
-        this._subscription = this._ngZone.runOutsideAngular(
-            () => this._contentObserver.observe(this._elementRef.nativeElement).subscribe(
-                () => this._liveAnnouncer.announce(
-                    this._elementRef.nativeElement.innerText, this._politeness)));
-      }
+    } else if (!this._subscription) {
+      this._subscription = this._ngZone.runOutsideAngular(() => {
+        return this._contentObserver
+          .observe(this._elementRef.nativeElement)
+          .subscribe(() => {
+            // Note that we use textContent here, rather than innerText, in order to avoid a reflow.
+            const element = this._elementRef.nativeElement;
+            this._liveAnnouncer.announce(element.textContent, this._politeness);
+          });
+      });
     }
   }
   private _politeness: AriaLivePoliteness = 'off';