Cleanup: Adding more conversion helper interfaces.

Source/web/WebDataSourceImpl.h

Index: Source/web/WebDataSourceImpl.h
diff --git a/Source/web/WebDataSourceImpl.h b/Source/web/WebDataSourceImpl.h
index 2c4cd8075122077894e03f3bbb3a6c7d21ba6c4c..8100ed47f39774f1c63409558c17bec19eb5fdb4 100644
--- a/Source/web/WebDataSourceImpl.h
+++ b/Source/web/WebDataSourceImpl.h
@@ -91,6 +91,13 @@ private:
     OwnPtr<WebPluginLoadObserver> m_pluginLoadObserver;
 };
 
+inline WebDataSourceImpl* toWebDataSourceImpl(WebDataSource* source)
+{
+    // WebDataSourceImpl is the only implementation of WebDataView, so
+    // no further checking required.
+    return static_cast<WebDataSourceImpl*>(source);
+}
+
 } // namespace WebKit
 
 #endif  // WebDataSourceImpl_h