Show the debug tab by default when the main isolate is paused

If I start a process with `dart --observe` and open devtools from the link in the output, I see a message saying `Main isolate paused in the debugger` (e.g. because of `--pause-isolates-on-start` or an uncaught exception). I'd prefer if devtools would open the Debugger tab if the main isolate rather than the performance tab.