Remove code duplication in AbstractPropertySetCSSStyleDeclaration::setProperty

This code is duplicated between setPropertyInternal and setProperty. We
should de-dupe it so we can add specific optimizations to the property
setting code for things like transform and opacity.

Review URL: https://codereview.chromium.org/214333002

git-svn-id: svn://svn.chromium.org/blink/trunk@170160 bbb929c8-8fbe-4397-9dbb-9b2b20218538