Add comments about threadSafeBind()

Source/platform/ThreadSafeFunctional.h

 // Copyright 2015 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 ThreadSafeFunctional_h
 #define ThreadSafeFunctional_h
 
 #include "platform/CrossThreadCopier.h"
 #include "wtf/Functional.h"
 
 namespace blink {
 
+// threadSafeBind() is bind() for cross-thread task posting.
+// threadSafeBind() applies CrossThreadCopier to the arguments.
+//
+// Example:
+//     void func1(int, const String&);
+//     f = threadSafeBind(func1, 42, str);
+// func1(42, str2) will be called when |f()| is executed,
+// where |str2| is a deep copy of |str| (created by str.isolatedCopy()).
+//
+// threadSafeBind(str) is similar to bind(str.isolatedCopy()), but the latter
+// is NOT thread-safe due to temporary objects (https://crbug.com/390851).
+//
+// Don't (if you pass the task across threads):
+//     bind(func1, 42, str);
+//     bind(func1, 42, str.isolatedCopy());
+
 template<typename... FreeVariableTypes, typename FunctionType>
 PassOwnPtr<Function<typename WTF::FunctionWrapper<FunctionType>::ResultType(FreeVariableTypes...)>> threadSafeBind(
     FunctionType function)
 {
     return bind<FreeVariableTypes...>(function);
 }
 
 template<typename... FreeVariableTypes, typename FunctionType, typename P1>
 PassOwnPtr<Function<typename WTF::FunctionWrapper<FunctionType>::ResultType(FreeVariableTypes...)>> threadSafeBind(
     FunctionType function,
@@ skipping 59 matching lines @@
         CrossThreadCopier<P2>::copy(parameter2),
         CrossThreadCopier<P3>::copy(parameter3),
         CrossThreadCopier<P4>::copy(parameter4),
         CrossThreadCopier<P5>::copy(parameter5),
         CrossThreadCopier<P6>::copy(parameter6));
 }
 
 } // namespace blink
 
 #endif // ThreadSafeFunctional_h