chore: cleanup ElementHandle.isIntersectingViewport (#2874)

This patch:
- simplifies test reusing the `offscreenbuttons.html` asset
- aligns IntersectionObserver usage with the one we have for
  `ElementHandle._scrollIntoViewIfNeeded`.

test/input.spec.js

@@ -33,7 +33,7 @@ module.exports.addTests = function({testRunner, expect, DeviceDescriptors}) {
       await page.goto(server.PREFIX + '/offscreenbuttons.html');
       const messages = [];
       page.on('console', msg => messages.push(msg.text()));
-      for (let i = 0; i < 10; ++i) {
+      for (let i = 0; i < 11; ++i) {
         // We might've scrolled to click a button - reset to (0, 0).
         await page.evaluate(() => window.scrollTo(0, 0));
         await page.click(`#btn${i}`);
@@ -48,7 +48,8 @@ module.exports.addTests = function({testRunner, expect, DeviceDescriptors}) {
         'button #6 clicked',
         'button #7 clicked',
         'button #8 clicked',
-        'button #9 clicked'
+        'button #9 clicked',
+        'button #10 clicked'
       ]);
     });