[ChromeDriver] Support mobile emulation on desktop Chrome.

chrome/test/chromedriver/mobile_device.cc

Index: chrome/test/chromedriver/mobile_device.cc
diff --git a/chrome/test/chromedriver/mobile_device.cc b/chrome/test/chromedriver/mobile_device.cc
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index 0000000000000000000000000000000000000000..769ef0eb1ce215b66f48e0dcac236c9acde5dcb6
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+++ b/chrome/test/chromedriver/mobile_device.cc
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+// 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/mobile_device.h"
+
+MobileDevice::MobileDevice() {}
+MobileDevice::MobileDevice(DeviceMetrics device_metrics, std::string user_agent)
+  : device_metrics(device_metrics), user_agent(user_agent) {}
+MobileDevice::~MobileDevice() {}