Don't spawn debugger.mojo by default in `mojo_run`.

mojo/devtools/common/README.md

 # Devtools
 
 Unopinionated tools for **running**, **debugging** and **testing** Mojo apps.
 
 ## Install
 
 ```
 git clone https://github.com/domokit/devtools.git
 ```
 
@@ skipping 39 matching lines @@
 By default, `mojo_run` uses https://core.mojoapps.io/kiosk_wm.mojo as the window
 manager. You can pass a different window manager url using the
 `--window-manager` flag to override this.
 
 ### Debugger
 
 `mojo_debug` allows you to interactively inspect a running shell, collect
 performance traces and attach a gdb debugger.
 
 #### Tracing
-To collect [performance
+In order to collect [performance
 traces](https://www.chromium.org/developers/how-tos/trace-event-profiling-tool)
-and retrieve the result:
+interactively through `mojo_debug`, make sure that the app being inspected was
+run with `--debugger` switch. E.g.:
+
+```sh
+mojo_run --debugger APP_URL [--android]
+```
+
+Then, in another shell, tracing can be orchestrated as follows:
 
 ```sh
 mojo_debug tracing start
 mojo_debug tracing stop [result.json]
 ```
 
 The trace file can be then loaded using the trace viewer in Chrome available at
 `about://tracing`.
 
 #### GDB
@@ skipping 34 matching lines @@
 ```sh
 mojo_debug device stack
 ```
 
 ## Development
 
 The library is canonically developed [in the mojo
 repository](https://github.com/domokit/mojo/tree/master/mojo/devtools/common),
 https://github.com/domokit/devtools is a mirror allowing to consume it
 separately.