chromeos: Remove "Found disk" warning messages.

Change DiskMountManagerImpl's "Found disk" warning messages
to DVLOG(1). I don't think that these indicate problems.

Bug: 826990
Change-Id: I60a8ae1e032d209f4649d774ee4fb60d91842197
Reviewed-on: https://chromium-review.googlesource.com/1014376Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Dan Erat <derat@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551245}

chromeos/disks/disk_mount_manager.cc

@@ -614,7 +614,7 @@ class DiskMountManagerImpl : public DiskMountManager,
     if (disk_info.is_virtual())
       return;
 
-    LOG(WARNING) << "Found disk " << disk_info.device_path();
+    DVLOG(1) << "Found disk " << disk_info.device_path();
     // Delete previous disk info for this path:
     bool is_new = true;
     std::string base_mount_path = std::string();