Move DeviceMonitorLinux to //device/core.

device/core/BUILD.gn

 # Copyright 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.
 
+import("//build/config/features.gni")
+
 component("core") {
   output_name = "device_core"
 
   sources = [
     "device_client.cc",
     "device_client.h",
     "device_info_query_win.cc",
     "device_info_query_win.h",
     "device_monitor_win.cc",
     "device_monitor_win.h",
   ]
 
   defines = [ "DEVICE_CORE_IMPLEMENTATION" ]
 
+  deps = []
+
   public_deps = [
     "//base",
   ]
 
+  if (use_udev) {
+    sources += [
+      "device_monitor_linux.cc",
+      "device_monitor_linux.h",
+    ]
+
+    deps += [ "//device/udev_linux" ]
+  }
+
   if (is_win) {
     libs = [ "setupapi.lib" ]
   }
 }
 
 source_set("mocks") {
   testonly = true
 
   sources = [
     "mock_device_client.cc",
     "mock_device_client.h",
   ]
 
   deps = [
     ":core",
     "//device/hid:mocks",
     "//device/usb:test_support",
   ]
 }