Connect mojo:debugger to window_manager only when requested.

This patch makes mojo:debugger automatically connect to window_manager
only when run by skydb, so that running the debugger for tracing and
profiling doesn't bring the side-effect of spawning window_manager.

Note that we might in the future change this to make the connection to
window_manager lazy when load() or reload() is requested, but this would
require a more involved change to skydb. The way taken in this patch
doesn't change the skydb codepath while enabling usage of debugger for
interactive tracing / profiling.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1157253007

tools/skydb

@@ -119,7 +119,7 @@ class SkyDebugger(object):
             '--v=1',
             '--content-handlers=%s' % ','.join(content_handlers),
             '--url-mappings=mojo:window_manager=mojo:kiosk_wm',
-            '--args-for=mojo:debugger %d' % remote_command_port,
+            '--args-for=mojo:debugger %d --wm' % remote_command_port,
             'mojo:debugger',
         ]