feat(elementhandle): introduce elementHandle.isIntersectingViewport() method. (#2673)

This patch introduces  `elementHandle.isIntersectingViewport()` method returns
true if element is visible in the viewport.

Fixes #2629.

lib/ElementHandle.js

@@ -354,6 +354,24 @@ class ElementHandle extends JSHandle {
     }
     return result;
   }
+
+  /**
+   * @returns {!Promise<boolean>}
+   */
+  isIntersectingViewport() {
+    return this.executionContext().evaluate(
+        node => new Promise(resolve => {
+          const callback = entries => {
+            resolve(entries[0].isIntersecting);
+            observer.disconnect();
+          };
+          const observer = new IntersectionObserver(callback);
+
+          observer.observe(node);
+        }),
+        this
+    );
+  }
 }
 
 function computeQuadArea(quad) {