net: Implement canceling of all async operations in FileStream.

content/browser/download/base_file.cc

Index: content/browser/download/base_file.cc
diff --git a/content/browser/download/base_file.cc b/content/browser/download/base_file.cc
index 9e055cef75a9effdde88038d71b38123a10adbfd..c7f6252fb14cf38844e2473b7c5125a0b07afb43 100644
--- a/content/browser/download/base_file.cc
+++ b/content/browser/download/base_file.cc
@@ -198,7 +198,8 @@ void BaseFile::Cancel() {
   if (!full_path_.empty()) {
     bound_net_log_.AddEvent(net::NetLog::TYPE_DOWNLOAD_FILE_DELETED);
 
-    file_util::Delete(full_path_, false);
+    bool deleted = file_util::Delete(full_path_, false);
+    DCHECK(deleted);
   }
 }
 
@@ -310,7 +311,6 @@ void BaseFile::Close() {
     // theres not much we can do.  But we might in the future.
     file_stream_->FlushSync();
 #endif
-    file_stream_->CloseSync();
     ClearStream();
   }
 }