[Repaint-after-layout] Remove triggering ASSERT

https://codereview.chromium.org/258693002/ landed an ASSERT
that happened to be triggering on some tests. The reason is
that for a layout where the root is not the view, we could
have positioned objects that belong to a containing block
higher in the tree than the layout root.

The optimization could be fixed but it would involve calling
isDescendantOf, which can be expensive (walking the tree up
to the root repeatedly), and it will get in the way of any
optimizations that avoids calling repaintTreeAfterLayout.
Thus it's more simple to just remove it.

Review URL: https://codereview.chromium.org/257943002

git-svn-id: svn://svn.chromium.org/blink/trunk@172785 bbb929c8-8fbe-4397-9dbb-9b2b20218538