Avoid tree walks when computing RenderLayer::scrollParent

This CL uses some memoization to avoid unnecessary tree walking when
computing scroll parent. Why memoization and not just passing down the
values? Because sad. It turns out that we recur on the stacking tree (we
have to - squashing demands processing stuff in paint order), but we
walk up the render tree asking questions. Different tree topologies; we
could very easily visit a RenderLayer before its great great
grandparent.

No functionality change -- covered by existing tests.
BUG=None

Review URL: https://codereview.chromium.org/206283003

git-svn-id: svn://svn.chromium.org/blink/trunk@169741 bbb929c8-8fbe-4397-9dbb-9b2b20218538