Add experiment to measure time to hash extension content as we read it

net/url_request/url_request_file_job.h

Index: net/url_request/url_request_file_job.h
diff --git a/net/url_request/url_request_file_job.h b/net/url_request/url_request_file_job.h
index a94596bd1fcd2c67c9b13d64646f07dcd36128c4..d6061be3979721e630f975600827876f2a07c49b 100644
--- a/net/url_request/url_request_file_job.h
+++ b/net/url_request/url_request_file_job.h
@@ -48,6 +48,10 @@ class NET_EXPORT URLRequestFileJob : public URLRequestJob {
   virtual void SetExtraRequestHeaders(
       const HttpRequestHeaders& headers) OVERRIDE;
 
+  // An interface for subclasses who wish to monitor read operations.
+  virtual void OnSeekComplete(int64 result);
+  virtual void OnReadComplete(net::IOBuffer* buf, int result);
+
  protected:
   virtual ~URLRequestFileJob();
 
@@ -88,8 +92,8 @@ class NET_EXPORT URLRequestFileJob : public URLRequestJob {
   // on a background thread.
   void DidSeek(int64 result);
 
-  // Callback after data is asynchronously read from the file.
-  void DidRead(int result);
+  // Callback after data is asynchronously read from the file into |buf|.
+  void DidRead(net::IOBuffer* buf, int result);
 
   scoped_ptr<FileStream> stream_;
   FileMetaInfo meta_info_;