Use nullptr_t for RefPtr, PassRefPtr and RawPtr.

Source/platform/fonts/mac/MemoryActivatedFont.mm

Index: Source/platform/fonts/mac/MemoryActivatedFont.mm
diff --git a/Source/platform/fonts/mac/MemoryActivatedFont.mm b/Source/platform/fonts/mac/MemoryActivatedFont.mm
index 3f3bbef007da1a75510142b7602762745fc70574..c96fe1e1e58563d6c803341c23d90b1af7ec3c9d 100644
--- a/Source/platform/fonts/mac/MemoryActivatedFont.mm
+++ b/Source/platform/fonts/mac/MemoryActivatedFont.mm
@@ -118,10 +118,10 @@ PassRefPtr<MemoryActivatedFont> loadFontFromBrowserProcess(NSFont* nsFont)
         // font due to being blocked by the sandbox.
         // This by definition shouldn't happen if there is no sandbox support.
         ASSERT_NOT_REACHED();
-        return 0;
+        return nullptr;
     }
     if (!sandboxSupport->loadFont(nsFont, &tmpCGFont, &fontID))
-        return 0;
+        return nullptr;
 
     RetainPtr<CGFontRef> cgFont(tmpCGFont);
     // Now that we have the fontID from the browser process, we can consult