Add support for gcm_sender_id in Manifest.

content/browser/manifest/manifest_manager_host.cc

Index: content/browser/manifest/manifest_manager_host.cc
diff --git a/content/browser/manifest/manifest_manager_host.cc b/content/browser/manifest/manifest_manager_host.cc
index 796d0a969379322babb810446cda4ce1a7567006..fb07bedd41b8124dc519c444fe30544d2dc29907 100644
--- a/content/browser/manifest/manifest_manager_host.cc
+++ b/content/browser/manifest/manifest_manager_host.cc
@@ -111,6 +111,10 @@ void ManifestManagerHost::OnRequestManifestResponse(
         manifest.short_name.is_null());
   if (!manifest.start_url.is_empty() && !manifest.start_url.is_valid())
     manifest.start_url = GURL();
+  manifest.gcm_sender_id = base::NullableString16(
+        manifest.gcm_sender_id.string().substr(
+            0, Manifest::kMaxIPCStringLength),
+        manifest.gcm_sender_id.is_null());
 
   callback->Run(manifest);
   callbacks->Remove(request_id);