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

base/file_util_proxy.h

Index: base/file_util_proxy.h
===================================================================
--- base/file_util_proxy.h	(revision 61078)
+++ base/file_util_proxy.h	(working copy)
@@ -138,7 +138,7 @@
 
   // Writes to a file. If |offset| is greater than the length of the file,
   // |false| is returned. On success, the file pointer is moved to position
-  // |offset + bytes_to_write| in the file. If The callback can be NULL.
+  // |offset + bytes_to_write| in the file. The callback can be NULL.
   static bool Write(
       scoped_refptr<MessageLoopProxy> message_loop_proxy,
       base::PlatformFile file,
@@ -169,9 +169,18 @@
   static bool Truncate(
       scoped_refptr<MessageLoopProxy> message_loop_proxy,
       base::PlatformFile file,
-      long long length,
+      int64 length,
       StatusCallback* callback);
 
+  // Truncates a file to the given length. If |length| is greater than the
+  // current length of the file, the file will be extended with zeroes.
+  // The callback can be NULL.
+  static bool Truncate(
+      scoped_refptr<MessageLoopProxy> message_loop_proxy,
+      const FilePath& path,
+      int64 length,
+      StatusCallback* callback);
+
   // Flushes a file. The callback can be NULL.
   static bool Flush(
       scoped_refptr<MessageLoopProxy> message_loop_proxy,