Deflake WebViewTest#testPageScroll

The test was flaky because it checks every 50ms after calling
pageUp(true) or pageDown(true). The page scroll offset on UI thread may
not change with 50ms.

Instead, a polling check for the new scroll offset is in place, until
the polling check times out.

BUG:crbug.com/534643
Change-Id: I25f9806b24b3f17a85fc5f5e43e2296a1793d38a
1 file changed