[DevTools] Instantiate singletone objects in WebInspector.Main._loaded.

This reduces the initialization order issues.

BUG=none

Review URL: https://codereview.chromium.org/749463002

git-svn-id: svn://svn.chromium.org/blink/trunk@185810 bbb929c8-8fbe-4397-9dbb-9b2b20218538