Make blink::CSSComputedStyleDeclaration::length update the render tree to avoid stale computed style.

BUG=413573

Review URL: https://codereview.chromium.org/575323002

git-svn-id: svn://svn.chromium.org/blink/trunk@182295 bbb929c8-8fbe-4397-9dbb-9b2b20218538