[iOS] Clear sign-in state on start-up.

This CL makes the sign-in tests more resilient:
* it clears the sign-in state on setup
* it commits pref changes on tear down to make sure that clearing the
last sign-in account is actually written to disk.

Bug: 961728

Change-Id: I833f3c888312c4a38ed13fa6bd9373989cf6dbb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1605914
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659037}

ios/chrome/test/app/signin_test_util.mm

@@ -149,6 +149,11 @@ bool SignOutAndClearAccounts() {
   browser_state->GetPrefs()->ClearPref(prefs::kGoogleServicesLastAccountId);
   browser_state->GetPrefs()->ClearPref(prefs::kGoogleServicesLastUsername);
 
+  // |SignOutAndClearAccounts()| is called during shutdown. Commit all pref
+  // changes to ensure that clearing the last signed in account is saved on disk
+  // in case Chrome crashes during shutdown.
+  browser_state->GetPrefs()->CommitPendingWrite();
+
   // Clear known identities.
   ios::ChromeIdentityService* identity_service =
       ios::GetChromeBrowserProvider()->GetChromeIdentityService();

ios/chrome/test/earl_grey/chrome_test_case.mm

@@ -200,6 +200,11 @@ const CFTimeInterval kDrainTimeout = 5;
 
   chrome_test_util::ResetSigninPromoPreferences();
   chrome_test_util::ResetMockAuthentication();
+
+  // Reset any remaining sign-in state from previous tests.
+  GREYAssert(chrome_test_util::SignOutAndClearAccounts(),
+             @"Failed to clean up the sign-in state before starting the test.");
+
   CHROME_EG_ASSERT_NO_ERROR([ChromeEarlGrey openNewTab]);
 }