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
1 file changed