Created a helper function "FindDiskBySourcePath" in DiskMountManager.

chrome/browser/chromeos/disks/disk_mount_manager.h

Index: chrome/browser/chromeos/disks/disk_mount_manager.h
diff --git a/chrome/browser/chromeos/disks/disk_mount_manager.h b/chrome/browser/chromeos/disks/disk_mount_manager.h
index df6dd185cd84b16ffe4b39f44da00bbbcdee7759..546c4b752ec64a9d58efdb0a19905a15a1272c4b 100644
--- a/chrome/browser/chromeos/disks/disk_mount_manager.h
+++ b/chrome/browser/chromeos/disks/disk_mount_manager.h
@@ -198,6 +198,10 @@ class DiskMountManager {
   // 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;