Fix scoped_refptr construction from NULL

This is a precursor to adding a new scoped_refptr(std::nullptr_t)
constructor. The implicit conversion from NULL to scoped_refptr<T>
causes a compilation error with the new constructor. Replace NULL with
nullptr in any files where this is a problem.

This CL was uploaded by git cl split.

R=boliu@chromium.org

Bug: 1018887
Change-Id: Ifb7b963111c36b594c482314ca294e5f2fdf970a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884519
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710147}

content/shell/android/shell_manager.cc

@@ -57,10 +57,7 @@ void JNI_ShellManager_LaunchShell(JNIEnv* env,
   ShellBrowserContext* browserContext =
       ShellContentBrowserClient::Get()->browser_context();
   GURL url(base::android::ConvertJavaStringToUTF8(env, jurl));
-  Shell::CreateNewWindow(browserContext,
-                         url,
-                         NULL,
-                         gfx::Size());
+  Shell::CreateNewWindow(browserContext, url, nullptr, gfx::Size());
 }
 
 void DestroyShellManager() {