Start work on adding content/browser to GN build.

third_party/power_gadget/BUILD.gn

Index: third_party/power_gadget/BUILD.gn
diff --git a/mojo/public/interfaces/service_provider/BUILD.gn b/third_party/power_gadget/BUILD.gn
similarity index 59%
copy from mojo/public/interfaces/service_provider/BUILD.gn
copy to third_party/power_gadget/BUILD.gn
index 990d3e9b6264ceac8ce9bc5e69e02dcf3db0bfd5..0a3f81ff215025a72ff6925b61431e7daee31c39 100644
--- a/mojo/public/interfaces/service_provider/BUILD.gn
+++ b/third_party/power_gadget/BUILD.gn
@@ -2,10 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//mojo/public/tools/bindings/mojom.gni")
+assert(is_win, "This only compiles on Windows")
 
-mojom("service_provider") {
+source_set("power_gadget") {
   sources = [
-    "service_provider.mojom",
+    "PowerGadgetLib.cpp",
+    "PowerGadgetLib.h",
   ]
 }