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/device.cc b/device.cc
index 2e644af..995aeeb 100644
--- a/device.cc
+++ b/device.cc
@@ -286,6 +286,12 @@
   return true;
 }
 
+void Device::DropConnection() {
+  SLOG(Device, 2) << __func__;
+  DestroyIPConfig();
+  SelectService(NULL);
+}
+
 void Device::DestroyIPConfig() {
   DisableIPv6();
   if (ipconfig_.get()) {