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 1b9b3c7ff4e77503723c5aa55c490d5f55e4b305..c627795e48c75b820c187a25b998d82fde190346 100644
--- a/content/public/common/manifest.h
+++ b/content/public/common/manifest.h
@@ -31,6 +31,10 @@ struct CONTENT_EXPORT Manifest {
   // Empty if the parsing failed or the field was not present.
   GURL start_url;
 
+  // This is an extension only used when PushMessaging Blink flag is enabled.
+  // 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.