Document weak pointer factory constructor requirement.

base/memory/weak_ptr.h

Index: base/memory/weak_ptr.h
diff --git a/base/memory/weak_ptr.h b/base/memory/weak_ptr.h
index 8a433922fdf0009f1bc4ed0a1a0caa146a798977..1230ead14bf76513cfbc63e22a855db35ac91787 100644
--- a/base/memory/weak_ptr.h
+++ b/base/memory/weak_ptr.h
@@ -16,6 +16,7 @@
 //
 //  class Controller {
 //   public:
+//    Controller() : weak_factory_(this) {}

[Nico, 2015/09/11 18:41:39]

It's not always `this`, is it? It's just the pointer the weak pointer handed out
by the factor points to, right? It could also be a pointer to e.g. a field of
this.

[scheib, 2015/09/11 19:40:29]

True, but in this example the factory vends Controller pointers.

 //    void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
 //    void WorkComplete(const Result& result) { ... }
 //   private: