Created a helper function "FindDiskBySourcePath" in DiskMountManager.

chrome/browser/chromeos/imageburner/burn_controller.cc

 // 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.
 
 #include "chrome/browser/chromeos/imageburner/burn_controller.h"
 
 #include "base/bind.h"
 #include "base/file_path.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/chromeos/cros/burn_library.h"
@@ skipping 316 matching lines @@
       CrosLibrary::Get()->GetBurnLibrary()->CancelBurnImage();
       CrosLibrary::Get()->GetBurnLibrary()->RemoveObserver(this);
       observing_burn_lib_ = false;
     }
     burn_manager_->ResetTargetPaths();
   }
 
   int64 GetDeviceSize(const std::string& device_path) {
     disks::DiskMountManager* disk_mount_manager =
         disks::DiskMountManager::GetInstance();
-    const disks::DiskMountManager::DiskMap& disks = disk_mount_manager->disks();
-    return disks.find(device_path)->second->total_size_in_bytes();
+    const disks::DiskMountManager::Disk* disk =
+        disk_mount_manager->FindDiskBySourcePath(device_path);
+    return disk ? disk->total_size_in_bytes() : 0;
   }
 
   bool CheckNetwork() {
     return CrosLibrary::Get()->GetNetworkLibrary()->Connected();
   }
 
   FilePath zip_image_file_path_;
   std::string image_file_name_;
   BurnManager* burn_manager_;
   StateMachine* state_machine_;
   bool observing_burn_lib_;
   bool working_;
   BurnController::Delegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(BurnControllerImpl);
 };
 
 }  // namespace
 
 // static
 BurnController* BurnController::CreateBurnController(
     content::WebContents* web_contents,
     Delegate* delegate) {
   return new BurnControllerImpl(delegate);
 }
 
 }  // namespace imageburner
 }  // namespace chromeos