OOBE: Fix discovery session logic on HIDDetectionScreen.

This fixes an issue where a discovery session is never started on the
HIDDetectionScreen, leaving the user unable to pair a Bluetooth input
device through the automatic connection flow.

Bug: b:168247012
Change-Id: I30f09ff661081afe0f71d8624d61cec10dc31093
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495832Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/master@{#821468}

chrome/browser/chromeos/login/screens/hid_detection_screen.cc

@@ -532,6 +532,8 @@ void HIDDetectionScreen::TryInitiateBTDevicesUpdate() {
                      weak_ptr_factory_.GetWeakPtr()),
           base::Bind(&HIDDetectionScreen::SetPoweredError,
                      weak_ptr_factory_.GetWeakPtr()));
+    } else if (!discovery_session_ || !discovery_session_->IsActive()) {
+      StartBTDiscoverySession();
     } else {
       UpdateBTDevices();
     }