Use "auto" in ArcAppResult::Duplicate()

One of the use-cases of auto is to remove duplicated type information:
  std::unique_ptr<X> x = std::make_unique<X>(...);
then becomes just
  auto x = std::make_unique<X>(...);
The benefit is code which is easier to read, without losing any
information about the type.

This CL is a follow-up after https://crrev.com/c/998194, which
introduced a line
  std::unique_ptr<ArcAppResult> copy = std::make_unique<ArcAppResult>(
That line is being shortened using auto.

Bug: 733662
Change-Id: I694330c283a453b5d2020c8ec237fafdf6572f20
Reviewed-on: https://chromium-review.googlesource.com/1011604Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550681}

chrome/browser/ui/app_list/search/arc_app_result.cc

@@ -59,7 +59,7 @@ void ArcAppResult::Open(int event_flags) {
 }
 
 std::unique_ptr<ChromeSearchResult> ArcAppResult::Duplicate() const {
-  std::unique_ptr<ArcAppResult> copy = std::make_unique<ArcAppResult>(
+  auto copy = std::make_unique<ArcAppResult>(
       profile(), app_id(), controller(),
       display_type() == ash::SearchResultDisplayType::kRecommendation);
   copy->set_title(title());