Connects PostMessage() for OOPIFs

mandoline/tab/frame_apptest.cc

Index: mandoline/tab/frame_apptest.cc
diff --git a/mandoline/tab/frame_apptest.cc b/mandoline/tab/frame_apptest.cc
index 32685a70058f63bee9dcc154079f966320c54fd6..8d7c7466f784668614c44513a920d5a28f236a61 100644
--- a/mandoline/tab/frame_apptest.cc
+++ b/mandoline/tab/frame_apptest.cc
@@ -153,6 +153,9 @@ class TestFrameTreeClient : public FrameTreeClient {
   void OnFrameClientPropertyChanged(uint32_t frame_id,
                                     const mojo::String& name,
                                     mojo::Array<uint8_t> new_data) override {}
+  void PostMessage(uint32_t source_frame_id,
+                   uint32_t target_frame_id,
+                   HTMLMessageEventPtr event) override {}
 
  private:
   int connect_count_;