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=seantopping@chromium.org

Bug: 1018887
Change-Id: I379afddc4e505e5d2260e3ec076cf19ed1ff46ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1885076
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710167}

chromecast/browser/cast_web_view_default.cc

@@ -42,7 +42,7 @@ namespace {
 std::unique_ptr<content::WebContents> CreateWebContents(
     content::BrowserContext* browser_context,
     scoped_refptr<content::SiteInstance> site_instance) {
-  content::WebContents::CreateParams create_params(browser_context, NULL);
+  content::WebContents::CreateParams create_params(browser_context, nullptr);
   create_params.routing_id = MSG_ROUTING_NONE;
   create_params.site_instance = site_instance;
   return content::WebContents::Create(create_params);