Increase Bind/Callback Arity from 6 -> 7.

base/bind.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
 $$
 $$ http://code.google.com/p/googletest/wiki/PumpManual
 $$
 
-$var MAX_ARITY = 6
+$$
+$$ MAX_ARITY controls the number of arguments that Bind() supports.
+$$ The amount of code, and more importantly, the number of template types
+$$ generated by pump grows at O(MAX_ARITY^2).
+$$
+$$ We tried going to 11 and found it imposed an extra 10 penalty on windows
+$$ cycle times compared to our original baseline of 6.
+$$
+$$ Currently 7 is chosen as a compromise between supporting a convenient
+$$ number of arguments and keeping compile times low.  At 7, we have 115
+$$ templates being generated by pump.
+$$
+$$ Be careful when adjusting this number.  If people find a need to bind
+$$ a larger number of arguments, consider refactoring the function to use
+$$ a param struct instead of raising the MAX_ARITY.
+$$
+$$ See http://crbug.com/98542 for more context.
+$$
+$var MAX_ARITY = 7
 
 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_BIND_H_
 #define BASE_BIND_H_
 #pragma once
 
 #include "base/bind_internal.h"
@@ skipping 94 matching lines @@
 void($for ARG , [[typename internal::CallbackParamTraits<P$(ARG)>::StorageType]])>(
           internal::MakeRunnable(functor)[[]]
 $if ARITY > 0 [[, ]] $for ARG , [[p$(ARG)]]));
 }
 
 ]]  $$ for ARITY
 
 }  // namespace base
 
 #endif  // BASE_BIND_H_