Make CSSValuePair store const CSSValues

Make CSSValuePair store const CSSValues, which involves making the
constructor take and getter methods return const CSSValue*s. This is an
entirely mechanical change that is more semantically correct and safe,
and is pre-work for making CSSStyleValue::toCSSValue() return a const
CSSValue.

BUG=526586

Review-Url: https://codereview.chromium.org/2158723002
Cr-Commit-Position: refs/heads/master@{#406459}
4 files changed