DevTools: StylePropertyTreeElementBase.updateTitle() refactoring

Extract nested function |processColor| into a method, so that it will be easier to enable the Closure compiler ES5 strict mode checks.

Review URL: https://codereview.chromium.org/106663004

git-svn-id: svn://svn.chromium.org/blink/trunk@163330 bbb929c8-8fbe-4397-9dbb-9b2b20218538