[iOS] Fix race condition in SignedInAccountsViewController.

The potential race condition occurs when the accounts with refresh
tokens becomes misaligned with the accounts in the Chrome Identity
service.

Bug: 1069950
Change-Id: Icdf3542ca69026f06a4cc8b793a0c2f1bc3518e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2375368
Commit-Queue: Nohemi Fernandez <fernandex@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801715}

ios/chrome/browser/ui/authentication/signed_in_accounts_view_controller.mm

@@ -119,6 +119,12 @@ BOOL gSignedInAccountsViewControllerIsShown = NO;
     ChromeIdentity* identity = ios::GetChromeBrowserProvider()
                                    ->GetChromeIdentityService()
                                    ->GetIdentityWithGaiaID(account.gaia);
+
+    // If the account with a refresh token is invalidated during this operation
+    // then |identity| will be nil. Do not process it in this case.
+    if (!identity) {
+      continue;
+    }
     CollectionViewItem* item = [self accountItem:identity];
     [model addItem:item toSectionWithIdentifier:SectionIdentifierAccounts];
     [mutableIdentityMap setObject:item forKey:identity.gaiaID];

ios/chrome/browser/ui/authentication/signin/signin_coordinator_egtest.mm

@@ -589,4 +589,26 @@ void ChooseImportOrKeepDataSepareteDialog(id<GREYMatcher> choiceButtonMatcher) {
       performAction:grey_tap()];
 }
 
+// Simulates a potential race condition in which the account is invalidated
+// after the user taps the Settings button to navigate to the identity
+// choosing UI. Depending on the timing, the account removal may occur after
+// the UI has retrieved the list of valid accounts. The test ensures that in
+// this case no account is presented to the user.
+- (void)testAccountInvalidatedDuringSignin {
+  FakeChromeIdentity* fakeIdentity = [SigninEarlGrey fakeIdentity1];
+  [SigninEarlGrey addFakeIdentity:fakeIdentity];
+
+  [ChromeEarlGreyUI openSettingsMenu];
+  [ChromeEarlGreyUI tapSettingsMenuButton:SecondarySignInButton()];
+
+  // Invalidate account after menu generation. If the underlying code does not
+  // handle the race condition of removing an identity while showing menu is in
+  // progress this test will likely be flaky.
+  [SigninEarlGrey forgetFakeIdentity:fakeIdentity];
+  // Check that the identity has been removed.
+  [[EarlGrey selectElementWithMatcher:identityChooserButtonMatcherWithEmail(
+                                          fakeIdentity.userEmail)]
+      assertWithMatcher:grey_notVisible()];
+}
+
 @end