Enable ash window cycle UI by default.

ash/common/wm/window_cycle_controller.cc

 // Copyright 2014 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 "ash/common/wm/window_cycle_controller.h"
 
 #include "ash/common/metrics/task_switch_source.h"
 #include "ash/common/session/session_state_delegate.h"
+#include "ash/common/shell_window_ids.h"
 #include "ash/common/wm/mru_window_tracker.h"
 #include "ash/common/wm/window_cycle_event_filter.h"
 #include "ash/common/wm/window_cycle_list.h"
 #include "ash/common/wm_shell.h"
+#include "ash/common/wm_window.h"
 #include "base/metrics/histogram.h"
 
 namespace ash {
 
 namespace {
 
 // Returns the most recently active window from the |window_list| or nullptr
 // if the list is empty.
 WmWindow* GetActiveWindow(const MruWindowTracker::WindowList& window_list) {
   return window_list.empty() ? nullptr : window_list[0];
@@ skipping 22 matching lines @@
 
   if (!IsCycling())
     StartCycling();
 
   Step(direction);
 }
 
 void WindowCycleController::StartCycling() {
   MruWindowTracker::WindowList window_list =
       WmShell::Get()->mru_window_tracker()->BuildMruWindowList();
+  // Exclude the AppList window, which will hide as soon as cycling starts
+  // anyway. It doesn't make sense to count it as a "switchable" window, yet
+  // a lot of code relies on the MRU list returning the app window. If we
+  // don't manually do this, the window cycling UI won't crash or misbehave, but
+  // there will be a flicker as the target window changes.

[Evan Stade, 2016/08/12 22:51:57]

this is really annoying and gross. See ash/common/wm/switchable_windows.{cc,h}.
In particular, the shelf's hide/show state depends on me not changing the
definition of "switchable" windows, even though the current definition is very
deceptive. There may be other places not covered by unit tests that also rely on
the app list being included in this window list as a lot of places seem to use
the MRU list.

+  window_list.erase(
+      std::remove_if(window_list.begin(), window_list.end(),
+                     [](WmWindow* window) {
+                       bool foo = window->GetRootWindow()
+                                      ->GetChildByShellWindowId(
+                                          kShellWindowId_AppListContainer)
+                                      ->Contains(window);
+                       return foo;
+                     }),
+      window_list.end());
 
   active_window_before_window_cycle_ = GetActiveWindow(window_list);
 
   window_cycle_list_.reset(new WindowCycleList(window_list));
   event_filter_ = WmShell::Get()->CreateWindowCycleEventFilter();
   cycle_start_time_ = base::Time::Now();
   WmShell::Get()->RecordUserMetricsAction(UMA_WINDOW_CYCLE);
   UMA_HISTOGRAM_COUNTS_100("Ash.WindowCycleController.Items",
                            window_list.size());
 }
@@ skipping 21 matching lines @@
 
   if (active_window_after_window_cycle != nullptr &&
       active_window_before_window_cycle_ != active_window_after_window_cycle) {
     WmShell::Get()->RecordTaskSwitchMetric(
         TaskSwitchSource::WINDOW_CYCLE_CONTROLLER);
   }
   active_window_before_window_cycle_ = nullptr;
 }
 
 }  // namespace ash