Prepare web_tests for Paint Holding Cross Origin

The Paint Holding Cross Origin feature extends Paint Holding to most
navigations. Paint Holding defers the first commit of rendering
data from the main thread to the compositor until after First
Contentful Paint, or a 500ms timeout, has occurred.

For web_tests there are issues with starting scroll animations
before the commit. So modify those tests a little to paint
something.

This also fixes issues with
css/css-animations/nested-scale-animations.html. It's behavior
is slightly different for some reason.

Bug: 1120158
Change-Id: I62c5e1533d4682488cfd5c29c1a0b47a77642135
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2773733


Auto-Submit: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864960}