Create a secondary file thread, FILE_USER_BLOCKING...

chrome/browser/browser_process.h

Index: chrome/browser/browser_process.h
===================================================================
--- chrome/browser/browser_process.h	(revision 111751)
+++ chrome/browser/browser_process.h	(working copy)
@@ -124,6 +124,10 @@
   // If you still must use this check the return value for NULL.
   virtual IOThread* io_thread() = 0;
 
+  // Returns the thread used for file system operations that block user
+  // interactions. Responsiveness of this thread affect users.
+  virtual base::Thread* file_user_blocking_thread() = 0;

[jam, 2011/11/29 17:31:09]

Joi is actually planning on removing the base::Thread getters for threads in
BrowserThread (since it interfers with having BrowserThread just be an interface
that content provides). It looks like this isn't needed, so please remove it

[michaeln, 2011/11/29 21:05:28]

Done.

+
   // Returns the thread that we perform random file operations on. For code
   // that wants to do I/O operations (not network requests or even file: URL
   // requests), this is the thread to use to avoid blocking the UI thread.