Decouple Power Monitor from //content.

device/power_monitor/public/cpp/BUILD.gn

+# 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")
+
+source_set("cpp") {
+  sources = [
+    "//device/power_monitor/power_monitor_export.h",
+    "power_monitor_broadcast_source.cc",
+    "power_monitor_broadcast_source.h",
+  ]
+
+  defines = [ "DEVICE_POWER_MONITOR_IMPLEMENTATION" ]
+
+  deps = [
+    "//base",
+    "//mojo/public/cpp/bindings",
+    "//services/service_manager/public/cpp",
+  ]
+
+  public_deps = [
+    "//device/power_monitor/public/interfaces",
+  ]
+}