Make StringKeyframe constructor store a const CSSValue

Make StringKeyframe constructor take a const CSSValue. This is an
entirely mechanical change and is pre-work to make
CSSPropertyParser::addProperty() take a const CSSValue&.

BUG=526586

Review-Url: https://codereview.chromium.org/2088683002
Cr-Commit-Position: refs/heads/master@{#404046}
1 file changed