Move MainWorldScriptContext accessor/method from WebFrame to WebLocalFrame.

content/shell/test_runner/test_runner_for_specific_view.h

Index: content/shell/test_runner/test_runner_for_specific_view.h
diff --git a/content/shell/test_runner/test_runner_for_specific_view.h b/content/shell/test_runner/test_runner_for_specific_view.h
index 863646118478257bb4b4bdfc81dc4800a15a0e51..6cb30a060c0eda61018b40f62a1adda84724bdd1 100644
--- a/content/shell/test_runner/test_runner_for_specific_view.h
+++ b/content/shell/test_runner/test_runner_for_specific_view.h
@@ -215,6 +215,11 @@ class TestRunnerForSpecificView {
   std::string SelectionAsMarkup();
   void SetViewSourceForFrame(const std::string& name, bool enabled);
 
+  // Many parts of the layout test harness assume that the main frame is local.
+  // Having all of them go through the helper below makes it easier to catch
+  // scenarios that require breaking this assumption.
+  blink::WebLocalFrame* GetLocalMainFrame();
+
   // Helpers for accessing pointers exposed by |web_view_test_proxy_base_|.
   blink::WebView* web_view();
   WebTestDelegate* delegate();