Fix invalid C-style cast in RenderFrameImplTest.

content/renderer/render_frame_impl_browsertest.cc

Index: content/renderer/render_frame_impl_browsertest.cc
diff --git a/content/renderer/render_frame_impl_browsertest.cc b/content/renderer/render_frame_impl_browsertest.cc
index ceb52cd626293343294bbb5938217a221e0e8bba..59850424af051d20f2127895def16483f6bf2b9c 100644
--- a/content/renderer/render_frame_impl_browsertest.cc
+++ b/content/renderer/render_frame_impl_browsertest.cc
@@ -110,7 +110,20 @@ class RenderFrameTestObserver : public RenderFrameObserver {
 // RenderWidget.
 TEST_F(RenderFrameImplTest, MAYBE_SubframeWidget) {
   EXPECT_TRUE(frame_widget());
-  EXPECT_NE(frame_widget(), (content::RenderWidget*)view_);
+  // We can't convert to content::RenderWidget* directly,
+  // because it and content::RenderView are two unrelated base classes

[nasko, 2015/08/06 20:55:20]

nit: drop the "content::" prefix, as this code is already in the content
namespace.

[krasin, 2015/08/06 21:02:22]

Done.

+  // of content::RenderViewImpl. If a class has multiple base classes,
+  // each base class pointer will be distinct, and direct casts
+  // between unrelated base classes are undefined, even if they share
+  // a common derived class. The compiler has no way in general of
+  // determining the displacement between the two classes, so these
+  // types of casts cannot be implemented in a type safe way.
+  // So overcome this, we make two legal static casts:

[nasko, 2015/08/06 20:55:20]

nit: s/So/To/

[krasin, 2015/08/06 21:02:22]

Done.

+  // first, downcast from RenderView* to RenderViewImpl*,
+  // then upcast from RenderViewImpl* to RenderWidger*.

[nasko, 2015/08/06 20:55:20]

nit: RenderWidget

[krasin, 2015/08/06 21:02:22]

Done.

+  EXPECT_NE(frame_widget(),
+            static_cast<content::RenderWidget*>(
+                static_cast<content::RenderViewImpl*>((view_))));
 }
 
 // Verify a subframe RenderWidget properly processes its viewport being