[DevTools] Move DevToolsWindow testing code to a separate class.

      This change:
- allows for proper docking testing (including toolbox);
- removes all test friends from DevToolsWindow;
- unifies testing approach to use load callback instead of notifications;
- adds simple toolbox tests.

BUG=393566

Review URL: https://codereview.chromium.org/395783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284423 0039d316-1c4b-4281-b951-d872f2087c98