[ChromeDriver] Support mobile emulation on desktop Chrome.

chrome/test/chromedriver/chrome/device_metrics.cc

+// Copyright (c) 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 "chrome/test/chromedriver/chrome/device_metrics.h"
+
+DeviceMetrics::DeviceMetrics(int width, int height, double device_scale_factor)
+  : width(width),
+    height(height),
+    device_scale_factor(device_scale_factor),
+    emulate_viewport(false),
+    fit_window(true),
+    text_autosizing(true),
+    font_scale_factor(1) {}
+
+DeviceMetrics::~DeviceMetrics() {}