Make MediaFileUtil run on thread pool.

webkit/fileapi/file_system_operation_context.h

Index: webkit/fileapi/file_system_operation_context.h
diff --git a/webkit/fileapi/file_system_operation_context.h b/webkit/fileapi/file_system_operation_context.h
index 3a8cc3931b0c2afbf6e3026b1a76d433ab554ac2..3e0ccd8334147baf9cf8deeb97414b21318288ac 100644
--- a/webkit/fileapi/file_system_operation_context.h
+++ b/webkit/fileapi/file_system_operation_context.h
@@ -50,7 +50,14 @@ class FILEAPI_EXPORT_PRIVATE FileSystemOperationContext {
   }
 #endif
 
-  base::SequencedTaskRunner* file_task_runner() const;
+  // Returns TaskRunner which the operation is performed on.
+  base::SequencedTaskRunner* task_runner() const {
+    return task_runner_.get();
+  }
+
+  // Overrides TaskRunner which the operation is performed on.
+  // file_system_context_->task_runners()->file_task_runner() is used otherwise.
+  void set_task_runner(base::SequencedTaskRunner* task_runner);
 
   void set_media_path_filter(MediaPathFilter* media_path_filter) {
     media_path_filter_ = media_path_filter;
@@ -62,6 +69,7 @@ class FILEAPI_EXPORT_PRIVATE FileSystemOperationContext {
 
  private:
   scoped_refptr<FileSystemContext> file_system_context_;
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
 
   int64 allowed_bytes_growth_;
   MediaPathFilter* media_path_filter_;