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=michaeldo@chromium.org

Bug: 1024981
Change-Id: I2d8b8c4b572199f76d40466074343c4fc6885870
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918151
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715467}

ios/web/webui/url_data_manager_ios_backend.mm

@@ -327,7 +327,7 @@ void URLRequestChromeJob::DataAvailable(base::RefCountedMemory* bytes) {
     if (pending_buf_.get()) {
       CHECK(pending_buf_->data());
       int rv = CompleteRead(pending_buf_.get(), pending_buf_size_);
-      pending_buf_ = NULL;
+      pending_buf_.reset();
       ReadRawDataComplete(rv);
     }
   } else {