Fix network service hanging with empty bodies.

mojo/public/cpp/bindings/sync_call_restrictions.h

Index: mojo/public/cpp/bindings/sync_call_restrictions.h
diff --git a/mojo/public/cpp/bindings/sync_call_restrictions.h b/mojo/public/cpp/bindings/sync_call_restrictions.h
index f83f3f0f5ee0fb18fb6e88817faca396bc560039..7ff8d1c0e077e689f154857522a281e5a19d6cc2 100644
--- a/mojo/public/cpp/bindings/sync_call_restrictions.h
+++ b/mojo/public/cpp/bindings/sync_call_restrictions.h
@@ -15,6 +15,10 @@
 #define ENABLE_SYNC_CALL_RESTRICTIONS 0
 #endif
 
+namespace content {
+class BrowserTestBase;
+}
+
 namespace display {
 class ForwardingDisplayDelegate;
 }
@@ -61,6 +65,8 @@ class MOJO_CPP_BINDINGS_EXPORT SyncCallRestrictions {
 #endif
 
  private:
+  // Test-only.

[yzshen1, 2017/06/05 21:07:04]

Does it make sense to move line 68-69 into the "BEGIN/END ALLOWED USAGE"
section?

[jam, 2017/06/06 00:15:47]

Done.

+  friend class content::BrowserTestBase;
   // DO NOT ADD ANY OTHER FRIEND STATEMENTS, talk to mojo/OWNERS first.
   // BEGIN ALLOWED USAGE.
   friend class ui::Gpu;  // http://crbug.com/620058