Re-enable testSignInCancelAddAccount earl grey test.

Sign-in UI code went through a massive architectural change which seems to
have fixed the test.

Bug: 962847
Change-Id: I1a37f8000a6792bce5f75a842376a1d30ff129f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106586Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751296}

ios/chrome/browser/ui/signin_interaction/signin_interaction_controller_egtest.mm

@@ -551,10 +551,7 @@ void ChooseImportOrKeepDataSepareteDialog(id<GREYMatcher> choiceButtonMatcher) {
 
 // Opens the add account screen and then cancels it by opening a new tab.
 // Ensures that the add account screen is correctly dismissed. crbug.com/462200
-//
-// TODO(crbug.com/962847): This test crashes when the the add account screen
-// is dismissed.
-- (void)DISABLED_testSignInCancelAddAccount {
+- (void)testSignInCancelAddAccount {
   // Add an identity to avoid arriving on the Add Account screen when opening
   // sign-in.
   FakeChromeIdentity* fakeIdentity = [SigninEarlGreyUtils fakeIdentity1];