Refactor BaseFile class to support sparse files

content/browser/download/save_file.cc

Index: content/browser/download/save_file.cc
diff --git a/content/browser/download/save_file.cc b/content/browser/download/save_file.cc
index 96cd0bf9447e82bd3f2f6ff44e4c11978ee17978..6d6337ce6dd6e98e954a376153b2bfb392e901f5 100644
--- a/content/browser/download/save_file.cc
+++ b/content/browser/download/save_file.cc
@@ -28,7 +28,8 @@ SaveFile::~SaveFile() {
 
 DownloadInterruptReason SaveFile::Initialize() {
   return file_.Initialize(base::FilePath(), base::FilePath(), base::File(), 0,
-                          std::string(), std::unique_ptr<crypto::SecureHash>());
+                          std::string(), std::unique_ptr<crypto::SecureHash>(),
+                          BaseFile::EXCLUSIVE);
 }
 
 DownloadInterruptReason SaveFile::AppendDataToFile(const char* data,