[iOS][MW] Fixing AppState will enter foreground

UIApplicationWillEnterForeground doesn't exist with MultiWindow enabled.

To keep the same behavior in Chrome, MainApplicationDelegate needs to
listen to UISceneWillEnterForegroundNotification notifications.
When the first window goes back to foreground, the method needs be
called:
-[AppState applicationWillEnterForeground:metricsMediator:memoryHelper:
tabOpener:]

Related to crbug.com/1097080:
"AuthenticationService should not rely on foreground/background
notifications"

Fixed: 1092485, 1090748
Change-Id: Ic248fa9c88a10bb3340b1a4f32efa75b9b751491
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252103
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781224}

ios/chrome/app/application_delegate/app_state.mm

@@ -319,7 +319,7 @@ initWithBrowserLauncher:(id<BrowserLauncher>)browserLauncher
     [self initializeUI];
     return;
   }
-  if ([self isInSafeMode])
+  if ([self isInSafeMode] || !_applicationInBackground)
     return;
 
   _applicationInBackground = NO;

ios/chrome/app/application_delegate/app_state_unittest.mm

@@ -708,6 +708,13 @@ TEST_F(AppStateTest, applicationWillEnterForeground) {
   [[[tabOpener stub] andReturnValue:@YES]
       shouldOpenNTPTabOnActivationOfBrowser:browser.get()];
 
+  // Simulate background before going to foreground.
+  [[getStartupInformationMock() expect] expireFirstUserActionRecorder];
+  swizzleMetricsMediatorDisableReporting();
+  [getAppStateWithMock() applicationDidEnterBackground:application
+                                          memoryHelper:memoryHelper
+                               incognitoContentVisible:YES];
+
   void (^swizzleBlock)() = ^{
   };
 
@@ -725,6 +732,7 @@ TEST_F(AppStateTest, applicationWillEnterForeground) {
   // Tests.
   EXPECT_OCMOCK_VERIFY(metricsMediator);
   EXPECT_OCMOCK_VERIFY(memoryHelper);
+  EXPECT_OCMOCK_VERIFY(getStartupInformationMock());
   FakeUserFeedbackProvider* user_feedback_provider =
       static_cast<FakeUserFeedbackProvider*>(
           ios::GetChromeBrowserProvider()->GetUserFeedbackProvider());

ios/chrome/app/main_application_delegate.mm

@@ -138,6 +138,11 @@
              selector:@selector(sceneDidEnterBackground:)
                  name:UISceneDidEnterBackgroundNotification
                object:nil];
+      [[NSNotificationCenter defaultCenter]
+          addObserver:self
+             selector:@selector(sceneWillEnterForeground:)
+                 name:UISceneWillEnterForegroundNotification
+               object:nil];
     }
   }
 
@@ -250,7 +255,7 @@
 - (void)sceneDidEnterBackground:(NSNotification*)notification {
   DCHECK(IsSceneStartupSupported());
   if (@available(iOS 13, *)) {
-    // When the first scene enters foreground, update the app state.
+    // When the last scene enters background, update the app state.
     if (self.foregroundSceneCount == 0) {
       [_appState applicationDidEnterBackground:UIApplication.sharedApplication
                                   memoryHelper:_memoryHelper
@@ -260,6 +265,19 @@
   }
 }
 
+- (void)sceneWillEnterForeground:(NSNotification*)notification {
+  DCHECK(IsSceneStartupSupported());
+  if (@available(iOS 13, *)) {
+    // When the first scene will enter foreground, update the app state.
+    if (self.foregroundSceneCount == 0) {
+      [_appState applicationWillEnterForeground:UIApplication.sharedApplication
+                                metricsMediator:_metricsMediator
+                                   memoryHelper:_memoryHelper
+                                      tabOpener:_tabOpener];
+    }
+  }
+}
+
 #pragma mark Downloading Data in the Background
 
 - (void)application:(UIApplication*)application