[DevTools] Cleanup InspectorView.

This change:
- moves overlay view to separate file - to be reused in undocked overlay;
- makes InspectorView to be what |InspectorView.devtoolsElement| was before;
- incapsulates logic specific for screencast or docking mode in separate places;
- allows multiple split views around overlay - to be used in emulation UI;
- makes screencast a separate frontend module.

BUG=none

Review URL: https://codereview.chromium.org/163953002

git-svn-id: svn://svn.chromium.org/blink/trunk@167786 bbb929c8-8fbe-4397-9dbb-9b2b20218538