[DevTools] Refactor docking code.

- Introduced status bar button type, which will be also used for screencast.
- Immediately change UI when dock state is changed, notifying
InspectorFrontendHost. We don't wait for setDockSide call anymore.
- Store dock state in frontend setting, and ignore "dock_side" parameter.

Next step is to switch from "dock_side" to "is_docked" parameter when
communicating with InspectorFrontendHost.

BUG=318751

Review URL: https://codereview.chromium.org/118223002

git-svn-id: svn://svn.chromium.org/blink/trunk@164292 bbb929c8-8fbe-4397-9dbb-9b2b20218538