Provide iOS implementation of screen.h.

ui/gfx/screen_ios.mm

Index: ui/gfx/screen_ios.mm
diff --git a/ui/gfx/screen_ios.mm b/ui/gfx/screen_ios.mm
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+// 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/gfx/screen.h"
+
+#import <UIKit/UIKit.h>
+
+#include "ui/gfx/display.h"
+
+namespace gfx {
+
+// static
+gfx::Display Screen::GetPrimaryDisplay() {
+  UIScreen* mainScreen = [[UIScreen screens] objectAtIndex:0];
+  gfx::Display display(0, gfx::Rect(mainScreen.bounds));
+  return display;
+}
+
+// static
+int Screen::GetNumDisplays() {
+#if TARGET_IPHONE_SIMULATOR
+  // UIScreen does not reliably return correct results on the simulator.
+  return 1;
+#else
+  return [[UIScreen screens] count];
+#endif
+}
+
+}  // namespace gfx