Make base::AutoReset moveable

This was brought up in the discussion of
https://crrev.com/c/1611240 that we should make it
moveable to avoid heap allocations when we have to
pass it around.

TBR=oshima@chromium.org
BUG=866622
TEST=base_unittests, and existing ash_unittests.

Change-Id: I7c6fb3c010c01129e6dcc8a0155bb53d35d295d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613371
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660112}

ash/wm/desks/desk.cc

@@ -110,10 +110,8 @@ void Desk::AddWindowToDesk(aura::Window* window) {
   NotifyContentChanged();
 }
 
-std::unique_ptr<base::AutoReset<bool>>
-Desk::GetScopedNotifyContentChangedDisabler() {
-  return std::make_unique<base::AutoReset<bool>>(
-      &should_notify_content_changed_, false);
+base::AutoReset<bool> Desk::GetScopedNotifyContentChangedDisabler() {
+  return base::AutoReset<bool>(&should_notify_content_changed_, false);
 }
 
 void Desk::Activate(bool update_window_activation) {

ash/wm/desks/desk.h

@@ -64,8 +64,7 @@ class ASH_EXPORT Desk : public aura::WindowObserver {
 
   void AddWindowToDesk(aura::Window* window);
 
-  std::unique_ptr<base::AutoReset<bool>>
-  GetScopedNotifyContentChangedDisabler();
+  base::AutoReset<bool> GetScopedNotifyContentChangedDisabler();
 
   // Activates this desk. All windows on this desk (if any) will become visible
   // (by means of showing this desk's associated containers on all root

base/BUILD.gn

@@ -2421,6 +2421,7 @@ test("base_unittests") {
     "android/unguessable_token_android_unittest.cc",
     "at_exit_unittest.cc",
     "atomicops_unittest.cc",
+    "auto_reset_unittests.cc",
     "barrier_closure_unittest.cc",
     "base64_unittest.cc",
     "base64url_unittest.cc",

base/auto_reset.h

@@ -29,7 +29,25 @@ class AutoReset {
     *scoped_variable_ = std::move(new_value);
   }
 
-  ~AutoReset() { *scoped_variable_ = std::move(original_value_); }
+  AutoReset(AutoReset&& other)
+      : scoped_variable_(other.scoped_variable_),
+        original_value_(std::move(other.original_value_)) {
+    other.scoped_variable_ = nullptr;
+  }
+
+  ~AutoReset() {
+    if (scoped_variable_)
+      *scoped_variable_ = std::move(original_value_);
+  }
+
+  AutoReset& operator=(AutoReset&& rhs) {
+    if (this != &rhs) {
+      scoped_variable_ = rhs.scoped_variable_;
+      rhs.scoped_variable_ = nullptr;
+      original_value_ = std::move(rhs.original_value_);
+    }
+    return *this;
+  }
 
  private:
   T* scoped_variable_;

base/auto_reset_unittests.cc

@@ -0,0 +1,33 @@
+// Copyright 2019 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.
+
+#include "base/auto_reset.h"
+
+#include <utility>
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+TEST(AutoReset, Move) {
+  int value = 10;
+  {
+    AutoReset<int> resetter1{&value, 20};
+    EXPECT_EQ(20, value);
+    {
+      value = 15;
+      AutoReset<int> resetter2 = std::move(resetter1);
+      // Moving to a new resetter does not change the value;
+      EXPECT_EQ(15, value);
+    }
+    // Moved-to `resetter2` is out of scoped, and resets to the original value
+    // that was in moved-from `resetter1`.
+    EXPECT_EQ(10, value);
+    value = 105;
+  }
+  // Moved-from `resetter1` does not reset to anything.
+  EXPECT_EQ(105, value);
+}
+
+}  // namespace base