Make CSSStyleValue::toCSSValue() return a const CSSValue*

Make CSSStyleValue::toCSSValue() return a const CSSValue*, and update
callsites to match (including updating Element::setInlineStyleProperty()
to take a const CSSValue*). This is an entirely mechanical change that
is more semantically correct and safe.

BUG=526586

Review-Url: https://codereview.chromium.org/2160483002
Cr-Commit-Position: refs/heads/master@{#406497}
8 files changed