[DevTools] Added WebInspector.VBox/HBox.

Box knows about it's own layout, which will be used in follow-up patches.
Most views became boxes.

BUG=none

Review URL: https://codereview.chromium.org/174053002

git-svn-id: svn://svn.chromium.org/blink/trunk@169006 bbb929c8-8fbe-4397-9dbb-9b2b20218538