Make applyMultiplier retain RenderStyle ptrs only during layout.

This should be a functional nop but slightly reduce memory impact of
resetMultipliers.

Review URL: https://codereview.chromium.org/268833008

git-svn-id: svn://svn.chromium.org/blink/trunk@173457 bbb929c8-8fbe-4397-9dbb-9b2b20218538