Screen overriding in tabs_api_unittest is fixed

TabsApiUnitTest overrides display::Screen and it left a screen
overridden after execution. When next test (not TabsApiUnitTest) from
current batch is executed and it uses display::Screen::GetScreen(),
it gets broken screen.

Bug: None
Change-Id: I15d3f0193b8980cc06ea0b5232e393e071738a7d
Reviewed-on: https://chromium-review.googlesource.com/963961Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544301}

chrome/browser/extensions/api/tabs/tabs_api_unittest.cc

@@ -26,6 +26,21 @@ namespace extensions {
 
 namespace {
 
+class ScopedScreenOverride {
+ public:
+  ~ScopedScreenOverride() {
+    display::Screen::SetScreenInstance(original_screen_);
+  }
+
+  void SetScreenInstance(display::Screen* instance) {
+    original_screen_ = display::Screen::GetScreen();
+    display::Screen::SetScreenInstance(instance);
+  }
+
+ private:
+  display::Screen* original_screen_ = nullptr;
+};
+
 std::unique_ptr<base::ListValue> RunTabsQueryFunction(
     Browser* browser,
     const Extension* extension,
@@ -58,6 +73,8 @@ class TabsApiUnitTest : public ExtensionServiceTestBase {
 
   display::test::TestScreen test_screen_;
 
+  std::unique_ptr<ScopedScreenOverride> scoped_screen_override_;
+
   DISALLOW_COPY_AND_ASSIGN(TabsApiUnitTest);
 };
 
@@ -71,12 +88,14 @@ void TabsApiUnitTest::SetUp() {
   params.type = Browser::TYPE_TABBED;
   params.window = browser_window_.get();
   browser_.reset(new Browser(params));
-  display::Screen::SetScreenInstance(&test_screen_);
+  scoped_screen_override_.reset(new ScopedScreenOverride);
+  scoped_screen_override_->SetScreenInstance(&test_screen_);
 }
 
 void TabsApiUnitTest::TearDown() {
   browser_.reset();
   browser_window_.reset();
+  scoped_screen_override_.reset();
   content::BrowserSideNavigationTearDown();
   ExtensionServiceTestBase::TearDown();
 }