Fix the flaky unit test BuiltInAppTest.Build.

The new app icon factory code in AppService calls
display::Screen::GetScreen to get the scale, so in unit tests, sometime,
it crashes:
0x7f3430a96390 (/lib/x86_64-linux-gnu/libpthread-2.23.so+0x1138f)
#4 0x558ca24e91ea (anonymous namespace)::GetPrimaryDisplayScaleFactor()

Modify unit tests to set the TestScreen. Built bots run 20 times, and
there is no flaky tests for BuiltInAppTest.Build.

BUG=1115837

Change-Id: Id3ad017f5e15dbadfc6b28ef0d65295bfcc799b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2379419
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802860}

chrome/browser/ui/app_list/app_service/app_service_app_model_builder_unittest.cc

@@ -61,8 +61,11 @@
 #include "extensions/browser/uninstall_reason.h"
 #include "extensions/common/extension_set.h"
 #include "extensions/common/manifest.h"
+#include "ui/display/test/scoped_screen_override.h"
+#include "ui/display/test/test_screen.h"
 
 using crostini::CrostiniTestHelper;
+using display::test::ScopedScreenOverride;
 using extensions::AppSorting;
 using extensions::ExtensionSystem;
 using plugin_vm::PluginVmTestHelper;
@@ -150,6 +153,8 @@ class AppServiceAppModelBuilderTest
       scoped_feature_list_.InitAndDisableFeature(
           features::kDesktopPWAsWithoutExtensions);
     }
+    scoped_screen_override_ =
+        std::make_unique<ScopedScreenOverride>(&test_screen_);
   }
 
   ~AppServiceAppModelBuilderTest() override {}
@@ -188,6 +193,8 @@ class AppServiceAppModelBuilderTest
   std::unique_ptr<AppServiceAppModelBuilder> builder_;
   std::unique_ptr<FakeAppListModelUpdater> model_updater_;
   std::unique_ptr<test::TestAppListControllerDelegate> controller_;
+  display::test::TestScreen test_screen_;
+  std::unique_ptr<ScopedScreenOverride> scoped_screen_override_;
 };
 
 class BuiltInAppTest : public AppServiceAppModelBuilderTest {