[ios] Add wait for testSignInSwitchAccountsAndKeepDataSeparate.

Bug: 989618
Change-Id: Ie826531bc0e891bcea5524cdecdc662692d0e2de
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732350
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683868}

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

@@ -6,6 +6,7 @@
 
 #import <EarlGrey/EarlGrey.h>
 
+#import "base/test/ios/wait_util.h"
 #import "ios/chrome/browser/ui/authentication/signin_earlgrey_utils_app_interface.h"
 #import "ios/public/provider/chrome/browser/signin/fake_chrome_identity.h"
 
@@ -45,6 +46,14 @@ GREY_STUB_CLASS_IN_APP_MAIN_QUEUE(SigninEarlGreyUtilsAppInterface)
   // Required to avoid any problem since the following test is not dependant
   // to UI, and the previous action has to be totally finished before going
   // through the assert.
+  GREYAssert(base::test::ios::WaitUntilConditionOrTimeout(
+                 base::test::ios::kWaitForActionTimeout,
+                 ^bool {
+                   NSString* primaryAccountGaiaID =
+                       [SignInEarlGreyUtilsAppInterface primaryAccountGaiaID];
+                   return primaryAccountGaiaID.length > 0;
+                 }),
+             @"Sign in did not complete.");
   [[GREYUIThreadExecutor sharedInstance] drainUntilIdle];
 
   NSString* primaryAccountGaiaID =