Move DeviceMonitorLinux to //device/core.

device/core/BUILD.gn

Index: device/core/BUILD.gn
diff --git a/device/core/BUILD.gn b/device/core/BUILD.gn
index 86bbb3b249332ed5c5aebf95fe2a02640044b7b7..f4c8e1837bdb79ed8953545b28b2c4037193d98a 100644
--- a/device/core/BUILD.gn
+++ b/device/core/BUILD.gn
@@ -2,6 +2,8 @@
 # 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"
 
@@ -16,10 +18,21 @@ component("core") {
 
   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" ]
   }