Change GetAllDisplays() in all screens to return a const vector&.

extensions/browser/api/system_display/system_display_apitest.cc

 // Copyright 2013 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 <stdint.h>
 
 #include <utility>
 
 #include "base/debug/leak_annotations.h"
 #include "base/macros.h"
@@ skipping 29 matching lines @@
  protected:
   // Overridden from display::Screen:
   gfx::Point GetCursorScreenPoint() override { return gfx::Point(); }
   bool IsWindowUnderCursor(gfx::NativeWindow window) override { return false; }
   gfx::NativeWindow GetWindowAtScreenPoint(const gfx::Point& point) override {
     return gfx::NativeWindow();
   }
   int GetNumDisplays() const override {
     return static_cast<int>(displays_.size());
   }
-  std::vector<display::Display> GetAllDisplays() const override {
+  const std::vector<display::Display>& GetAllDisplays() const override {
     return displays_;
   }
   display::Display GetDisplayNearestWindow(
       gfx::NativeView window) const override {
     return display::Display(0);
   }
   display::Display GetDisplayNearestPoint(
       const gfx::Point& point) const override {
     return display::Display(0);
   }
@@ skipping 238 matching lines @@
     enable_unified_function->set_extension(test_extension.get());
     ASSERT_TRUE(api_test_utils::RunFunction(enable_unified_function.get(),
                                             "[false]", browser_context()));
     EXPECT_FALSE(provider_->unified_desktop_enabled());
   }
 }
 
 #endif  // !defined(OS_CHROMEOS)
 
 }  // namespace extensions