Add support for gcm_sender_id in Manifest.

content/public/common/manifest.h

Index: content/public/common/manifest.h
diff --git a/content/public/common/manifest.h b/content/public/common/manifest.h
index 957f0074fd3a38f9eca173c4f932c737bb241f84..038bd0d9f7c26d66a14972779b2ea29d139ddee4 100644
--- a/content/public/common/manifest.h
+++ b/content/public/common/manifest.h
@@ -82,6 +82,11 @@ struct CONTENT_EXPORT Manifest {
   // icons inside the JSON array were invalid.
   std::vector<Icon> icons;
 
+  // This is a proprietary extension of the web Manifest, double-check that it
+  // is okay to use this entry.
+  // Null if parsing failed or the field was not present.
+  base::NullableString16 gcm_sender_id;
+
   // Maximum length for all the strings inside the Manifest when it is sent over
   // IPC. The renderer process should truncate the strings before sending the
   // Manifest and the browser process must do the same when receiving it.