Start work on adding content/browser to GN build.

content/public/browser/BUILD.gn

Index: content/public/browser/BUILD.gn
diff --git a/mojo/spy/ui/__init__.py b/content/public/browser/BUILD.gn
similarity index 71%
copy from mojo/spy/ui/__init__.py
copy to content/public/browser/BUILD.gn
index bfecdb26df6584f5627a9ec6e7da764ce16a4ce5..a258390cc3ecfb667e896257bad8cf62315e2cdb 100644
--- a/mojo/spy/ui/__init__.py
+++ b/content/public/browser/BUILD.gn
@@ -2,4 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-from ui import tvcm_stub
+source_set("browser") {
+  deps = [
+    "//content/browser",
+  ]
+}