drive: Fix a crash caused by an empty edit URL

chrome/browser/chromeos/gdata/drive_file_system.h

Index: chrome/browser/chromeos/gdata/drive_file_system.h
diff --git a/chrome/browser/chromeos/gdata/drive_file_system.h b/chrome/browser/chromeos/gdata/drive_file_system.h
index 4179dff411cb84423358e9b8bd1427b32ed78f99..3143b98b74d52ebdc602ef09d1b1260e0a488c1e 100644
--- a/chrome/browser/chromeos/gdata/drive_file_system.h
+++ b/chrome/browser/chromeos/gdata/drive_file_system.h
@@ -489,7 +489,7 @@ class DriveFileSystem : public DriveFileSystemInterface,
 
   // Callback for DriveServiceInterface::DeleteDocument. Removes the entry with
   // |resource_id| from the local snapshot of the filesystem and the cache.
-  // |document_url| is unused. |callback| may be null.
+  // |document_url| is unused. |callback| must not be null.
   void RemoveResourceLocally(const FileOperationCallback& callback,
                              const std::string& resource_id,
                              GDataErrorCode status,
@@ -780,6 +780,7 @@ class DriveFileSystem : public DriveFileSystemInterface,
 
   // Part of RemoveOnUIThread(). Called after GetEntryInfoByPath() is
   // complete.
+  // |callback| must not be null.
   void RemoveOnUIThreadAfterGetEntryInfo(
       const FileOperationCallback& callback,
       DriveFileError error,