Run clang-modernize -use-nullptr over src/extensions/.

extensions/browser/app_window/app_web_contents_helper.cc

Index: extensions/browser/app_window/app_web_contents_helper.cc
diff --git a/extensions/browser/app_window/app_web_contents_helper.cc b/extensions/browser/app_window/app_web_contents_helper.cc
index fb6647937e5fadeb41ee913e8ae2b49c862c91f4..d418f2a07ce9c8c3636b15b63c38bb8e312fd6f3 100644
--- a/extensions/browser/app_window/app_web_contents_helper.cc
+++ b/extensions/browser/app_window/app_web_contents_helper.cc
@@ -51,13 +51,13 @@ content::WebContents* AppWebContentsHelper::OpenURLFromTab(
         base::StringPrintf(
             "Can't open same-window link to \"%s\"; try target=\"_blank\".",
             params.url.spec().c_str()));
-    return NULL;
+    return nullptr;
   }
 
   // These dispositions aren't really navigations.
   if (disposition == SUPPRESS_OPEN || disposition == SAVE_TO_DISK ||
       disposition == IGNORE_ACTION) {
-    return NULL;
+    return nullptr;
   }
 
   content::WebContents* contents =