Remove dependency on RenderStyle when creating CSSPrimitiveValues from Lengths

RenderStyles were used in the conversion from Lengths to
CSSPrimitiveValues to undo the zoom that gets applied to Lengths. Only
the zoom value associated with the Length is required to do this rather
than an entire RenderStyle object.

This patch is in preparation for converting animation snapshot Lengths
back to CSS without needing to store an entire RenderStyle object to know
the zoom value.

Review URL: https://codereview.chromium.org/26709005

git-svn-id: svn://svn.chromium.org/blink/trunk@159401 bbb929c8-8fbe-4397-9dbb-9b2b20218538