[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
DevTools: merge TracingTimelineModel into TimelineModel

Now that there is only one implementation it doesn't make sense to separate interface from implementation of TimelineModel.

BUG=428700

Review URL: https://codereview.chromium.org/715803002

git-svn-id: svn://svn.chromium.org/blink/trunk@185130 bbb929c8-8fbe-4397-9dbb-9b2b20218538