DevTools: Extract the WebInspector namespace into WebInspector.js

The trivial code with no dependencies goes into WebInspector.js, too.
The externs.js file has been cleaned up accordingly.

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/187573004

git-svn-id: svn://svn.chromium.org/blink/trunk@169265 bbb929c8-8fbe-4397-9dbb-9b2b20218538