device/bluetooth: Remove DCHECK(IsDiscovering()) until BlueZ is fixed.

BlueZ bug makes it wrong to assume that the Discovering property is
updated when StartDiscovery/StopDiscovery returns (crbug.com/822104).
BlueZ bug fixing is deprioritized so here we remove the DCHECK first
until BlueZ bug is fixed.

BUG=814595
TEST=No longer crashes

Change-Id: I12c6d0df808553d3098525b70ab998d8c957b918
Reviewed-on: https://chromium-review.googlesource.com/963845Reviewed-by: Miao-chen Chou <mcchou@chromium.org>
Commit-Queue: Sonny Sasaka <sonnysasaka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543495}

device/bluetooth/bluez/bluetooth_adapter_bluez.cc

@@ -1383,7 +1383,9 @@ void BluetoothAdapterBlueZ::AddDiscoverySession(
 
   // The adapter is already discovering.
   if (num_discovery_sessions_ > 0) {
-    DCHECK(IsDiscovering());
+    // DCHECK(IsDiscovering()) is removed due to BlueZ bug
+    // (https://crbug.com/822104).
+    // TODO(sonnysasaka): Put it back here when BlueZ bug is fixed.
     DCHECK(!discovery_request_pending_);
     num_discovery_sessions_++;
     SetDiscoveryFilter(BluetoothDiscoveryFilter::Merge(