crostini: Delay the CrosUsbDetector ConnectToDeviceManager call.

DeviceManager isn't necessarily ready to be used straight away. It
should be ready after PostBrowserStart returns.

Bug: 1129735
Change-Id: I9b65ab48a3b442bf2803a0337354b8fefaab38d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530962Reviewed-by: Timothy Loh <timloh@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Auto-Submit: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826293}

chrome/browser/chromeos/chrome_browser_main_chromeos.cc

@@ -1082,7 +1082,10 @@ void ChromeBrowserMainPartsChromeos::PostBrowserStart() {
 
   // Enable Chrome OS USB detection.
   cros_usb_detector_ = std::make_unique<CrosUsbDetector>();
-  cros_usb_detector_->ConnectToDeviceManager();
+  content::GetUIThreadTaskRunner({base::TaskPriority::BEST_EFFORT})
+      ->PostTask(FROM_HERE,
+                 base::BindOnce(&CrosUsbDetector::ConnectToDeviceManager,
+                                base::Unretained(cros_usb_detector_.get())));
 
   crostini_unsupported_action_notifier_ =
       std::make_unique<crostini::CrostiniUnsupportedActionNotifier>();