Decouple Power Monitor from //content.

device/power_monitor/BUILD.gn

Index: device/power_monitor/BUILD.gn
diff --git a/device/power_monitor/BUILD.gn b/device/power_monitor/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..a799e01aad291c430bcb0353871ee08b096a7704
--- /dev/null
+++ b/device/power_monitor/BUILD.gn
@@ -0,0 +1,24 @@
+# Copyright 2016 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("power_monitor") {
+  sources = [
+    "power_monitor_export.h",
+    "power_monitor_message_broadcaster.cc",
+    "power_monitor_message_broadcaster.h",
+  ]
+
+  defines = [ "DEVICE_POWER_MONITOR_IMPLEMENTATION" ]
+
+  deps = [
+    "//base",
+    "//mojo/public/cpp/bindings",
+  ]
+
+  public_deps = [
+    "//device/power_monitor/public/interfaces",
+  ]
+}