Add GN Build file for extensions_browser target.

BUG=None
TEST=gn gen out/Debug_gn && ninja -C out/Debug_gn extensions/browser
[email protected]
[email protected]

Review URL: https://codereview.chromium.org/418763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285800 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/extensions/extensions.gyp b/extensions/extensions.gyp
index ece94767..edfa455 100644
--- a/extensions/extensions.gyp
+++ b/extensions/extensions.gyp
@@ -234,6 +234,7 @@
       ],
     },
     {
+      # GN version: //extensions/browser
       'target_name': 'extensions_browser',
       'type': 'static_library',
       'dependencies': [