[AW] Fix presubmit errors: use nullptr and make_unique.

This patch addresses the following issues:
 - presubmit prefers to have a nullptr instead of "std::unique_ptr<T>()".
 - also use make_unique instead of creating unique_ptr directly (via a
   pointer created with new), which is also preferred by presubmit.

BUG=993812

Change-Id: I930f235dad3d6b9f93670b9ec957d0bc71889ea4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1754163
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686854}

android_webview/browser/aw_contents_io_thread_client.cc

@@ -436,20 +436,21 @@ std::unique_ptr<AwWebResourceResponse> RunShouldInterceptRequest(
   RecordInterceptedScheme(ret.is_null(), request.url);
 
   if (ret.is_null())
-    return std::unique_ptr<AwWebResourceResponse>(nullptr);
+    return nullptr;
 
-  AwWebResourceResponse* response = new AwWebResourceResponse(ret);
+  std::unique_ptr<AwWebResourceResponse> response =
+      std::make_unique<AwWebResourceResponse>(ret);
   if (!response->HasInputStream(env)) {
     // Only record UMA for cases where the input stream is null (see
     // crbug.com/974273).
-    RecordResponseStatusCode(env, response);
+    RecordResponseStatusCode(env, response.get());
   }
 
-  return std::unique_ptr<AwWebResourceResponse>(response);
+  return response;
 }
 
 std::unique_ptr<AwWebResourceResponse> ReturnNull() {
-  return std::unique_ptr<AwWebResourceResponse>();
+  return nullptr;
 }
 
 }  // namespace

android_webview/browser/network_service/aw_web_resource_response.cc

@@ -42,13 +42,13 @@ std::unique_ptr<InputStream> AwWebResourceResponse::GetInputStream(
   DCHECK(!input_stream_transferred_);
 
   if (input_stream_transferred_)
-    return std::unique_ptr<InputStream>();
+    return nullptr;
 
   input_stream_transferred_ = true;
   ScopedJavaLocalRef<jobject> jstream =
       Java_AwWebResourceResponse_getData(env, java_object_);
   if (jstream.is_null())
-    return std::unique_ptr<InputStream>();
+    return nullptr;
   return std::make_unique<InputStream>(jstream);
 }