Removed the non-const version of PropertyReference::value()

Removed the version of PropertyReference::value() that returns a
non-const CSSValue*. This is an entirely mechanical change that is more
semantically correct and safe, and also removes a FIXME in the code.

BUG=526586

Review-Url: https://codereview.chromium.org/2046243002
Cr-Commit-Position: refs/heads/master@{#406463}
7 files changed