chromeos: Makes AshTestBase/Helper target mash when appropriate

ash/display/window_tree_host_manager_unittest.cc

 // Copyright (c) 2012 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/display/window_tree_host_manager.h"
 
 #include <memory>
 
 #include "ash/common/shelf/shelf_widget.h"
 #include "ash/common/shelf/wm_shelf.h"
@@ skipping 167 matching lines @@
   int activation_changed_count_;
 
   DISALLOW_COPY_AND_ASSIGN(TestObserver);
 };
 
 display::Display GetPrimaryDisplay() {
   return display::Screen::GetScreen()->GetDisplayNearestWindow(
       Shell::GetAllRootWindows()[0]);
 }
 
-display::Display GetSecondaryDisplay() {

[msw, 2017/02/19 21:49:20]

Can you give some context for this? Do we need this on AshTestBase/Helper for
parity with AshTest? (if so, why is it needed in this CL? afaict, nothing
changes base classes here; is this just to prep for those changes?)
Does GetPrimaryDisplay also belong there? (It'd be nice to see more consistency
with those or just rely on display::Screen or similar)

[sky, 2017/02/21 17:06:50]

Exactly.
I added GetSecondaryDisplay() to AshTestBase, which manes this override is no
longer necessary.
Shell has a couple of functions for the primary window, but not the secondary.
It's common for tests to want the secondary, hence the convenience function.

-  return display::Screen::GetScreen()->GetDisplayNearestWindow(
-      Shell::GetAllRootWindows()[1]);
-}
-
 class TestHelper {
  public:
   explicit TestHelper(test::AshTestBase* delegate);
   ~TestHelper();
 
   void SetSecondaryDisplayLayoutAndOffset(
       display::DisplayPlacement::Position position,
       int offset);
 
   void SetSecondaryDisplayLayout(display::DisplayPlacement::Position position);
@@ skipping 1511 matching lines @@
       widget->GetNativeWindow()->GetRootWindow());
   event_generator.MoveMouseToCenterOf(widget->GetNativeWindow());
 
   UpdateDisplay("300x300");
   watcher.Stop();
 
   widget->CloseNow();
 }
 
 }  // namespace ash