This is the IPC and bits of the browser backend for FileWriter. The rest of...

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 60387)
+++ chrome/browser/file_system/file_system_dispatcher_host.cc	(working copy)
@@ -115,6 +115,9 @@
     IPC_MESSAGE_HANDLER(ViewHostMsg_FileSystem_Create, OnCreate)
     IPC_MESSAGE_HANDLER(ViewHostMsg_FileSystem_Exists, OnExists)
     IPC_MESSAGE_HANDLER(ViewHostMsg_FileSystem_ReadDirectory, OnReadDirectory)
+    IPC_MESSAGE_HANDLER(ViewHostMsg_FileSystem_Write, OnWrite)
+    IPC_MESSAGE_HANDLER(ViewHostMsg_FileSystem_Truncate, OnTruncate)
+    IPC_MESSAGE_HANDLER(ViewHostMsg_FileSystem_CancelWrite, OnCancel)
     IPC_MESSAGE_UNHANDLED(handled = false)
   IPC_END_MESSAGE_MAP_EX()
   return handled;
@@ -206,6 +209,39 @@
   GetNewOperation(request_id)->ReadDirectory(path);
 }
 
+void FileSystemDispatcherHost::OnWrite(
+    int request_id,
+    const FilePath& path,
+    const GURL& blob_url,
+    int64 offset) {
+  if (!CheckValidFileSystemPath(path, request_id))
+    return;
+  GetNewOperation(request_id)->Write(path, blob_url, offset);
+}
+
+void FileSystemDispatcherHost::OnTruncate(
+    int request_id,
+    const FilePath& path,
+    int64 length) {
+  if (!CheckValidFileSystemPath(path, request_id))
+    return;
+  GetNewOperation(request_id)->Truncate(path, length);
+}
+
+void FileSystemDispatcherHost::OnCancel(
+    int request_id,
+    int request_id_to_cancel) {
+  fileapi::FileSystemOperation* write =
+      operations_.Lookup(request_id_to_cancel);
+  if (write) {
+    write->Cancel();
+    Send(new ViewMsg_FileSystem_DidSucceed(request_id));
+  } else {
+    Send(new ViewMsg_FileSystem_DidFail(
+        request_id, base::PLATFORM_FILE_ERROR_INVALID_OPERATION));
+  }
+}
+
 void FileSystemDispatcherHost::Send(IPC::Message* message) {
   DCHECK(ChromeThread::CurrentlyOn(ChromeThread::IO));
   if (!shutdown_ && message_sender_)