[fsp] Improve performance for reading small chunks of data.

webkit/browser/blob/blob_url_request_job.cc

Index: webkit/browser/blob/blob_url_request_job.cc
diff --git a/webkit/browser/blob/blob_url_request_job.cc b/webkit/browser/blob/blob_url_request_job.cc
index 9eb196ad40181c50ed19ad9dbf600205e1200b15..2cd201385a72ee950d46f15d79095efc13fa8f96 100644
--- a/webkit/browser/blob/blob_url_request_job.cc
+++ b/webkit/browser/blob/blob_url_request_job.cc
@@ -4,7 +4,10 @@
 
 #include "webkit/browser/blob/blob_url_request_job.h"
 
+#include <algorithm>
 #include <limits>
+#include <string>
+#include <vector>
 
 #include "base/basictypes.h"
 #include "base/bind.h"
@@ -13,6 +16,7 @@
 #include "base/format_macros.h"
 #include "base/message_loop/message_loop.h"
 #include "base/message_loop/message_loop_proxy.h"
+#include "base/numerics/safe_conversions.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
@@ -223,8 +227,8 @@ void BlobURLRequestJob::DidCountSize(int error) {
     return;
   }
 
-  remaining_bytes_ = byte_range_.last_byte_position() -
-                     byte_range_.first_byte_position() + 1;
+  remaining_bytes_ = base::checked_cast<int64>(
+      byte_range_.last_byte_position() - byte_range_.first_byte_position() + 1);
   DCHECK_GE(remaining_bytes_, 0);
 
   // Do the seek at the beginning of the request.
@@ -577,6 +581,7 @@ void BlobURLRequestJob::CreateFileStreamReader(size_t index,
                                              file_system_context_->CrackURL(
                                                  item.filesystem_url())),
                                          item.offset() + additional_offset,
+                                         item.length() - additional_offset,
                                          item.expected_modification_time())
                    .release();
       break;