Update WeakPtrFactory examples to use in-class initializers.

This is now the more common, and simpler, pattern.

Bug: 981415
Change-Id: I082e75425a33f6256c2ddc3228d61f8b6fe8bc58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1704677Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677960}

base/memory/weak_ptr.h

@@ -16,14 +16,13 @@
 //
 //  class Controller {
 //   public:
-//    Controller() : weak_factory_(this) {}
 //    void SpawnWorker() { Worker::StartNew(weak_factory_.GetWeakPtr()); }
 //    void WorkComplete(const Result& result) { ... }
 //   private:
 //    // Member variables should appear before the WeakPtrFactory, to ensure
 //    // that any WeakPtrs to Controller are invalidated before its members
 //    // variable's destructors are executed, rendering them invalid.
-//    WeakPtrFactory<Controller> weak_factory_;
+//    WeakPtrFactory<Controller> weak_factory_{this};
 //  };
 //
 //  class Worker {

docs/callback.md

@@ -355,13 +355,13 @@ To use `base::WeakPtr` with `base::Bind()`, `MyClass` will typically look like:
 ```cpp
 class MyClass {
 public:
-  MyClass() : weak_factory_(this) {
+  MyClass() {
     weak_this_ = weak_factory_.GetWeakPtr();
   }
 private:
   base::WeakPtr<MyClass> weak_this_;
   // MyClass member variables go here.
-  base::WeakPtrFactory<MyClass> weak_factory_;
+  base::WeakPtrFactory<MyClass> weak_factory_{this};
 };
 ```
 

docs/threading_and_tasks.md

@@ -588,8 +588,6 @@ int Compute() { … }
 
 class A {
  public:
-  A() : weak_ptr_factory_(this) {}
-
   void ComputeAndStore() {
     // Schedule a call to Compute() in a thread pool followed by
     // a call to A::Store() on the current sequence. The call to
@@ -604,7 +602,7 @@ class A {
   void Store(int value) { value_ = value; }
 
   int value_;
-  base::WeakPtrFactory<A> weak_ptr_factory_;
+  base::WeakPtrFactory<A> weak_ptr_factory_{this};
 };
 ```