Made StylePropertySet::getPropertyCSSValue return a const CSSValue*

Made StylePropertySet::getPropertyCSSValue return a const CSSValue*,
which is more semantically correct and safe. This also forces callers
to make copies of any CSSValues they want to edit before saving them
back to the style, making it clearer and more explicit when changes to
the style are made.

BUG=526586

Review-Url: https://codereview.chromium.org/2035373002
Cr-Commit-Position: refs/heads/master@{#405966}
11 files changed