devtools: DebuggerAgent interface

- added RuntimeAgent and IndexedDBAgent with limited functionality in
  the process in order to avoid warnings
- fixed securityOrigin not sent for getResourceTree
- storing scripts by id

Still encountering the following problems

- we should see Debugger.getScriptSource after replying to
  Page.getResourceContent, but don't

Somehow when setting a breakpoint it is unable to find the breakpoint
target although it knows about the script and it's source (sends along
scriptID and got resourceContent of that ID before).

The backend sends back an ok with the info about the breakpoint set and
at this point the UI tries to add it but has trouble mapping it to a
UILocation.
As a result the breakpoint never gets set properly and we are also
unable to remove it.

Stacktrace:

```
Uncaught TypeError: Cannot read property '_rawLocationToUILocation' of null Main.js:formatted:22893
WebInspector.DebuggerWorkspaceBinding.rawLocationToUILocation Main.js:formatted:22893
WebInspector.BreakpointManager.TargetBreakpoint._addResolvedLocation Main.js:formatted:26244
WebInspector.BreakpointManager.TargetBreakpoint._didSetBreakpointInDebugger Main.js:formatted:26219
didSetBreakpoint Main.js:formatted:22445InspectorBackendClass.AgentPrototype.dispatchResponse Main.js:formatted:5262
InspectorBackendClass.Connection.dispatch Main.js:formatted:5071
InspectorBackendClass.WebSocketConnection._onMessage
```