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=sadrul@chromium.org

Bug: 1024981
Change-Id: Ia7d9cd0d76794292be90704b14682ba656c45996
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918148
Auto-Submit: kylechar <kylechar@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716476}

ui/accelerated_widget_mac/window_resize_helper_mac.cc

@@ -167,7 +167,7 @@ void WrappedTask::RemoveFromTaskRunnerQueue() {
     pumpable_task_runner_->task_queue_.erase(iterator_);
     iterator_ = pumpable_task_runner_->task_queue_.end();
   }
-  pumpable_task_runner_ = NULL;
+  pumpable_task_runner_.reset();
 }
 
 ////////////////////////////////////////////////////////////////////////////////

ui/compositor/layer_animator_unittest.cc

@@ -1706,7 +1706,7 @@ TEST(LayerAnimatorTest, AddObserverExplicit) {
 
   animator->StartAnimation(sequence);
 
-  animator = NULL;
+  animator.reset();
 
   EXPECT_EQ(observer.last_aborted_sequence(), sequence);
 }
@@ -2366,7 +2366,7 @@ TEST(LayerAnimatorTest, ImplicitObserversAtAnimatorDestruction) {
 
   EXPECT_FALSE(observer_notify.animations_completed());
   EXPECT_FALSE(observer_do_not_notify.animations_completed());
-  animator = NULL;
+  animator.reset();
   EXPECT_TRUE(observer_notify.animations_completed());
   EXPECT_TRUE(observer_notify.WasAnimationAbortedForProperty(
       LayerAnimationElement::BRIGHTNESS));

ui/compositor/test/in_process_context_factory.cc

@@ -336,7 +336,7 @@ InProcessContextFactory::SharedMainThreadContextProvider() {
       &gpu_memory_buffer_manager_, &image_factory_, support_locking);
   auto result = shared_main_thread_contexts_->BindToCurrentThread();
   if (result != gpu::ContextResult::kSuccess)
-    shared_main_thread_contexts_ = NULL;
+    shared_main_thread_contexts_.reset();
 
   return shared_main_thread_contexts_;
 }

ui/gfx/image/image_skia.cc

@@ -497,7 +497,7 @@ void ImageSkia::RemoveUnsupportedRepresentationsForScale(float scale) {
 
 void ImageSkia::Init(const ImageSkiaRep& image_rep) {
   if (image_rep.GetBitmap().drawsNothing()) {
-    storage_ = NULL;
+    storage_.reset();
     return;
   }
   storage_ = new internal::ImageSkiaStorage(