Make PropertyReference value() return a const CSSValue&

Make PropertyReference value() return a const CSSValue& instead of a
const CSSValue*. Since a Property cannot have a null CSSValue, this is
more semantically correct and safe.

BUG=526586

Review-Url: https://codereview.chromium.org/2044023005
Cr-Commit-Position: refs/heads/master@{#406526}
16 files changed