Reduce duplication for converting font enums to CSS

This patch makes convertions for font enums in CSSComputedStyleDeclaration
just use the conversions in CSSPrimitiveValueMappings to reduce code
duplication.

The font-weights 400 and 700 are identical to normal and bolder (resp.),
so I've updated the CSSPrimitiveValueMappings conversion to match what
we currently do in CSSComputedStyleDeclaration.

Review URL: https://codereview.chromium.org/429953005

git-svn-id: svn://svn.chromium.org/blink/trunk@179288 bbb929c8-8fbe-4397-9dbb-9b2b20218538