DevTools: Code clean up in CodeGeneratorInspector.py
* Get rid of unused methods
* Get rid of code duplication
* Ease return values: Classes with a static string field are replaced with a string

BUG=

Review URL: https://codereview.chromium.org/558593003

git-svn-id: svn://svn.chromium.org/blink/trunk@181967 bbb929c8-8fbe-4397-9dbb-9b2b20218538