[ObjC ARC] Converts ios/chrome/browser/ui/activity_services:unit_tests to ARC.

ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator_unittest.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/ui/activity_services/chrome_activity_item_thumbnail_generator.h"
 
-#include "base/mac/scoped_nsobject.h"
 #import "ios/chrome/browser/browser_state/test_chrome_browser_state.h"
 #include "ios/chrome/browser/tabs/tab.h"
 #include "testing/platform_test.h"
 #import "third_party/ocmock/OCMock/OCMock.h"
 #include "ui/base/test/ios/ui_image_test_utils.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 class ChromeActivityItemThumbnailGeneratorTest : public PlatformTest {
  protected:
   void SetUp() override {
-    tab_.reset([[OCMockObject niceMockForClass:[Tab class]] retain]);
+    tab_ = [OCMockObject niceMockForClass:[Tab class]];
 
     UIImage* snapshot = ui::test::uiimage_utils::UIImageWithSizeAndSolidColor(
         CGSizeMake(400, 300), [UIColor redColor]);
 
     [[[tab_ stub] andReturn:snapshot] generateSnapshotWithOverlay:NO
                                                  visibleFrameOnly:YES];
   }
 
   void SetBrowserStateOnTab(bool incognito) {
     TestChromeBrowserState::Builder test_cbs_builder;
     chrome_browser_state_ = test_cbs_builder.Build();
     ios::ChromeBrowserState* browserState =
         incognito ? chrome_browser_state_->GetOffTheRecordChromeBrowserState()
                   : chrome_browser_state_.get();
     [[[tab_ stub] andReturnValue:OCMOCK_VALUE(browserState)] browserState];
   }
 
   std::unique_ptr<TestChromeBrowserState> chrome_browser_state_;
-  base::scoped_nsobject<id> tab_;
+  id tab_;
 };
 
 TEST_F(ChromeActivityItemThumbnailGeneratorTest, ThumbnailForNonIncognitoTab) {
   SetBrowserStateOnTab(false);
   CGSize size = CGSizeMake(50, 50);
   ThumbnailGeneratorBlock generatorBlock =
       activity_services::ThumbnailGeneratorForTab(tab_);
   EXPECT_TRUE(generatorBlock);
   UIImage* thumbnail = generatorBlock(size);
   EXPECT_TRUE(thumbnail);
   EXPECT_TRUE(CGSizeEqualToSize(thumbnail.size, size));
 }
 
 TEST_F(ChromeActivityItemThumbnailGeneratorTest, NoThumbnailForIncognitoTab) {
   SetBrowserStateOnTab(true);
   CGSize size = CGSizeMake(50, 50);
   ThumbnailGeneratorBlock generatorBlock =
       activity_services::ThumbnailGeneratorForTab(tab_);
   EXPECT_TRUE(generatorBlock);
   UIImage* thumbnail = generatorBlock(size);
   EXPECT_FALSE(thumbnail);
 }
 
 }  // namespace