Fix assigning NULL to scoped_refptr

Update code where NULL or 0 is assigned to a scoped_refptr to reset it. Just
call reset() instead. If operator=(std::nullptr_t) is added these assignments
become ambiguous.

This CL was uploaded by git cl split.

R=rmcilroy@chromium.org

Bug: 1024981
Change-Id: Id9096057c47c70ad94e97ddb42984c71e0ab078e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918319
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715682}

gin/array_buffer.cc

@@ -146,7 +146,7 @@ void ArrayBuffer::Private::FirstWeakCallback(
 void ArrayBuffer::Private::SecondWeakCallback(
     const v8::WeakCallbackInfo<Private>& data) {
   Private* parameter = data.GetParameter();
-  parameter->self_reference_ = NULL;
+  parameter->self_reference_.reset();
 }
 
 // ArrayBuffer ----------------------------------------------------------------