Add truncate operation to File API.

runtime/bin/file.h

Index: runtime/bin/file.h
diff --git a/runtime/bin/file.h b/runtime/bin/file.h
index 382adffa7b760320cd7b11d41bfd0ea470831a94..239544774e7c33de3ae2983e39d7881a8978f86d 100644
--- a/runtime/bin/file.h
+++ b/runtime/bin/file.h
@@ -46,7 +46,12 @@ class File {
   // Get the current position in the file.
   // Returns a negative value if position cannot be determined.
   off_t Position();
-  off_t SetPosition(int64_t position);
+
+  // Set the byte position in the file.
+  bool SetPosition(int64_t position);
+
+  // Truncate (or extend) the file to the given length in bytes.
+  bool Truncate(int64_t length);
 
   // Flush contents of file.
   void Flush();