Created a helper function "FindDiskBySourcePath" in DiskMountManager.

chrome/browser/chromeos/extensions/file_browser_event_router.cc

Index: chrome/browser/chromeos/extensions/file_browser_event_router.cc
diff --git a/chrome/browser/chromeos/extensions/file_browser_event_router.cc b/chrome/browser/chromeos/extensions/file_browser_event_router.cc
index ed931c3293be4707094799dfca2cedb6028d8a2a..89ea5447751e492aa70d0be13f2ba4bbb3cb4842 100644
--- a/chrome/browser/chromeos/extensions/file_browser_event_router.cc
+++ b/chrome/browser/chromeos/extensions/file_browser_event_router.cc
@@ -298,12 +298,10 @@ void FileBrowserEventRouter::MountCompleted(
   if (mount_info.mount_type == chromeos::MOUNT_TYPE_DEVICE &&
       event_type == DiskMountManager::MOUNTING) {
     DiskMountManager* disk_mount_manager = DiskMountManager::GetInstance();
-    DiskMountManager::DiskMap::const_iterator disk_it =
-        disk_mount_manager->disks().find(mount_info.source_path);
-    if (disk_it == disk_mount_manager->disks().end()) {
+    DiskMountManager::Disk* disk =
+        disk_mount_manager->FindDiskBySourcePath(mount_info.source_path);
+    if (!disk)
       return;
-    }
-    DiskMountManager::Disk* disk = disk_it->second;
 
     notifications_->ManageNotificationsOnMountCompleted(
         disk->system_path_prefix(), disk->drive_label(), disk->is_parent(),