Move ViewHostMsg_CreateFullscreenWidget to mojom.

content/public/test/mock_render_thread.cc

Index: content/public/test/mock_render_thread.cc
diff --git a/content/public/test/mock_render_thread.cc b/content/public/test/mock_render_thread.cc
index 35fd8dfcd7f54991b4182d1928a855e6518f368a..ad39d25d7aa57c71e83eb9a26d620f06c74d4648 100644
--- a/content/public/test/mock_render_thread.cc
+++ b/content/public/test/mock_render_thread.cc
@@ -69,6 +69,12 @@ class MockRenderMessageFilterImpl : public mojom::RenderMessageFilter {
     return true;
   }
 
+  void CreateFullscreenWidget(
+      int opener_id,
+      const CreateFullscreenWidgetCallback& callback) override {
+    NOTREACHED();
+  }
+
  private:
   MockRenderThread* const thread_;
 };