shill: wifi: Monitor the presence of WPA supplicant.

Connect to WPA supplicant only if it's present and suspend the device when WPA
supplicant disappears. This ensures that the WiFi device/service state remains
consistent across crashes of WPA supplicant, for example.

Also, drop the device connection when stopping the WiFi device. This avoids a
DCHECK for connections not being present in Manager::DeregisterService.

Remove a device restart on scan failure -- this should be covered now by the
explicit wpa_supplicant monitor.

BUG=chromium-os:31712,chromium-os:25657
TEST=tested on device, kill -SEGV wpa_supplicant; unit tests

Change-Id: I11efb1d841d4c7abd40da725b1d074170baee726
Reviewed-on: https://gerrit.chromium.org/gerrit/26017
Commit-Ready: Darin Petkov <petkov@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/dbus_manager_unittest.cc b/dbus_manager_unittest.cc
index 91e523b..db5bbc0 100644
--- a/dbus_manager_unittest.cc
+++ b/dbus_manager_unittest.cc
@@ -33,7 +33,7 @@
  protected:
   class TestProxyFactory : public ProxyFactory {
    public:
-    TestProxyFactory(DBusManagerTest *test) : test_(test) {}
+    explicit TestProxyFactory(DBusManagerTest *test) : test_(test) {}
 
     virtual DBusServiceProxyInterface *CreateDBusServiceProxy() {
       return test_->proxy_.release();