Add truncate and cancel for FileWriter; write and tests will come in later CL...

chrome/browser/file_system/file_system_dispatcher_host.cc

Index: chrome/browser/file_system/file_system_dispatcher_host.cc
===================================================================
--- chrome/browser/file_system/file_system_dispatcher_host.cc	(revision 61078)
+++ chrome/browser/file_system/file_system_dispatcher_host.cc	(working copy)
@@ -238,9 +238,11 @@
   fileapi::FileSystemOperation* write =
       operations_.Lookup(request_id_to_cancel);
   if (write) {
-    write->Cancel();
-    Send(new ViewMsg_FileSystem_DidSucceed(request_id));
+    // The cancel will eventually send both the write failure and the cancel
+    // success.
+    write->Cancel(GetNewOperation(request_id));
   } else {
+    // The write already finished; report that we failed to stop it.
     Send(new ViewMsg_FileSystem_DidFail(
         request_id, base::PLATFORM_FILE_ERROR_INVALID_OPERATION));
   }