webkit: deflake testRequestChildRectangleOnScreen. WebViewTest.testRequestChildRectangleOnScreen was flaking occasionally on emulators because the scroll position was not being updated immediately and the test was observing the old value. It's unclear why this happens, but using a polling check makes the test reliable. Bug: 334932789 Test: atest WebViewTest#testRequestChildRectangleOnScreen Change-Id: If4b45ddd7e99cf2a4fc9d716a292e4c29f634dbf