Turn styleWillChange |newStyle| into a reference

The parameter was never NULL but the code wasn't clear about it.
This lead to a couple of unneeded NULL-checks that obfuscated
what could happen.

Review URL: https://codereview.chromium.org/217563003

git-svn-id: svn://svn.chromium.org/blink/trunk@170482 bbb929c8-8fbe-4397-9dbb-9b2b20218538