Make chrome.filesystem API fully work with the Chrome OS Google Drive folder.

chrome/browser/extensions/api/file_system/file_system_api.h

Index: chrome/browser/extensions/api/file_system/file_system_api.h
diff --git a/chrome/browser/extensions/api/file_system/file_system_api.h b/chrome/browser/extensions/api/file_system/file_system_api.h
index e8013b7ace26ffd0a86db7ffcc021c1e583f5978..35f251d24b59dac5cc7d1f0a55d1b4ac3b4e7d17 100644
--- a/chrome/browser/extensions/api/file_system/file_system_api.h
+++ b/chrome/browser/extensions/api/file_system/file_system_api.h
@@ -32,9 +32,10 @@ class FileSystemEntryFunction : public AsyncExtensionFunction {
 
   bool HasFileSystemWritePermission();
 
-  // Called on the FILE thread. This is called when a writable file entry is
-  // being returned. The function will ensure the file exists, creating it if
-  // necessary, and also check that the file is not a link.
+  // This is called when a writable file entry is being returned. The function
+  // will ensure the file exists, creating it if necessary, and also check that
+  // the file is not a link. If it succeeds it proceeds to
+  // RegisterFileSystemAndSendResponse, otherwise to HandleWritableFileError.
   void CheckWritableFile(const base::FilePath& path);
 
   // This will finish the choose file process. This is either called directly