Created a helper function "FindDiskBySourcePath" in DiskMountManager.

chrome/browser/chromeos/disks/disk_mount_manager.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_CHROMEOS_DISKS_DISK_MOUNT_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_DISKS_DISK_MOUNT_MANAGER_H_
 
 #include <map>
 
 #include "chromeos/dbus/cros_disks_client.h"
@@ skipping 180 matching lines @@
 
   // Adds an observer.
   virtual void AddObserver(Observer* observer) = 0;
 
   // Removes an observer.
   virtual void RemoveObserver(Observer* observer) = 0;
 
   // Gets the list of disks found.
   virtual const DiskMap& disks() const = 0;
 
+  // Returns Disk object corresponding to |source_path| or NULL on failure.
+  virtual const Disk* FindDiskBySourcePath(
+      const std::string& source_path) const = 0;
+
   // Gets the list of mount points.
   virtual const MountPointMap& mount_points() const = 0;
 
   // Requests refreshing all the information about mounted disks.
   virtual void RequestMountInfoRefresh() = 0;
 
   // Mounts a device.
   virtual void MountPath(const std::string& source_path,
                          const std::string& source_format,
                          const std::string& mount_label,
@@ skipping 46 matching lines @@
 
   // Returns a pointer to the global DiskMountManager instance.
   // Initialize() should already have been called.
   static DiskMountManager* GetInstance();
 };
 
 }  // namespace disks
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_DISKS_DISK_MOUNT_MANAGER_H_