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

extensions/browser/extension_web_contents_observer.cc

Index: extensions/browser/extension_web_contents_observer.cc
diff --git a/extensions/browser/extension_web_contents_observer.cc b/extensions/browser/extension_web_contents_observer.cc
index a66a6f2d9459f3f25baa2e05cbc2b075390db68a..f46af6ddb69234dab4d20b2a394e75d74e89bd5a 100644
--- a/extensions/browser/extension_web_contents_observer.cc
+++ b/extensions/browser/extension_web_contents_observer.cc
@@ -98,7 +98,7 @@ const Extension* ExtensionWebContentsObserver::GetExtension(
     content::RenderViewHost* render_view_host) {
   std::string extension_id = GetExtensionId(render_view_host);
   if (extension_id.empty())
-    return NULL;
+    return nullptr;
 
   // May be null if the extension doesn't exist, for example if somebody typos
   // a chrome-extension:// URL.