[DevTools] Extract part of DevTools into inspector app.

This inspector app is intended to be run inside an iframe.
InspectorAppHost exposes an API between inspector app and hosting app.

Created DevTools app which is only embedding inspector app in iframe.
Next patches will define API and move hosting functionality from
inspector app to devtools app.

BUG=431697

Review URL: https://codereview.chromium.org/710033003

git-svn-id: svn://svn.chromium.org/blink/trunk@185127 bbb929c8-8fbe-4397-9dbb-9b2b20218538