Mac HiDPI: Fix scale factor on external display

ui/base/layout_mac.mm

 // 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 "ui/base/layout.h"
 
 #include <Cocoa/Cocoa.h>
 
 @interface NSScreen (LionAPI)
 - (CGFloat)backingScaleFactor;
 @end
 
 @interface NSWindow (LionAPI)
 - (CGFloat)backingScaleFactor;
 @end
 
 namespace {
 
 std::vector<ui::ScaleFactor>& GetSupportedScaleFactorsInternal() {
   static std::vector<ui::ScaleFactor>* supported_scale_factors =
       new std::vector<ui::ScaleFactor>();
   if (supported_scale_factors->empty()) {
     supported_scale_factors->push_back(ui::SCALE_FACTOR_100P);
     supported_scale_factors->push_back(ui::SCALE_FACTOR_200P);
   }
   return *supported_scale_factors;
 }
 
 float GetScaleFactorScaleForNativeView(gfx::NativeView view) {
-  float scale_factor = 1.0f;
   if (NSWindow* window = [view window]) {
     if ([window respondsToSelector:@selector(backingScaleFactor)])
       return [window backingScaleFactor];
-    scale_factor = [window userSpaceScaleFactor];
+    return [window userSpaceScaleFactor];
   }
-  if (NSScreen* screen = [NSScreen mainScreen]) {
-    if ([screen respondsToSelector:@selector(backingScaleFactor)])
-      return [screen backingScaleFactor];
-    return [screen userSpaceScaleFactor];
-  }
-  return 1.0f;
+
+  NSArray* screens = [NSScreen screens];
+  if (![screens count])
+    return 1.0f;
+
+  NSScreen* screen = [screens objectAtIndex:0];
+  if ([screen respondsToSelector:@selector(backingScaleFactor)])
+    return [screen backingScaleFactor];
+  return [screen userSpaceScaleFactor];
 }
 
 }  // namespace
 
 namespace ui {
 
 ScaleFactor GetScaleFactorForNativeView(gfx::NativeView view) {
   return GetScaleFactorFromScale(GetScaleFactorScaleForNativeView(view));
 }
 
 std::vector<ScaleFactor> GetSupportedScaleFactors() {
   return GetSupportedScaleFactorsInternal();
 }
 
 namespace test {
 
 void SetSupportedScaleFactors(
     const std::vector<ui::ScaleFactor>& scale_factors) {
   std::vector<ui::ScaleFactor>& supported_scale_factors =
       GetSupportedScaleFactorsInternal();
   supported_scale_factors = scale_factors;
 }
 
 }  // namespace test
 
 }  // namespace ui