Moved signin_test_util#SignOutAndClearAccounts to chrome_earl_grey

This change requires for /ios_internal repository(submit only after this cl)
https://chrome-internal-review.googlesource.com/c/chrome/ios_internal/+/1284283

This is part of EG1 to EG2 migration, which involves moving EarlGrey code from
app-side helpers into test code. For GREYAsserts, the policy is to take assert-
containing-helpers, modify them to return NSError* with localizedDescription
containing the assert failure (in the event of a failure), and then assert
on the result of the helper on the test side.

Bug: 922813
Change-Id: I42086810e5efe23139210fa1e198c14665f903bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606248
Commit-Queue: Maksym Onufriienko <monufriienko@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659657}

ios/chrome/test/earl_grey/chrome_earl_grey.h

@@ -159,6 +159,13 @@ id ExecuteJavaScript(NSString* javascript, NSError* __autoreleasing* out_error);
 // Sets value for content setting.
 + (NSError*)setContentSettings:(ContentSetting)setting WARN_UNUSED_RESULT;
 
+#pragma mark - Sign Utilities
+
+// Signs the user out, clears the known accounts entirely and checks whether
+// the accounts were correctly removed from the keychain. Returns nil on
+// success, or else an NSError indicating why the operation failed.
++ (NSError*)signOutAndClearAccounts WARN_UNUSED_RESULT;
+
 @end
 
 #endif  // IOS_CHROME_TEST_EARL_GREY_CHROME_EARL_GREY_H_

ios/chrome/test/earl_grey/chrome_earl_grey.mm

@@ -26,6 +26,7 @@
 #import "ios/chrome/test/app/history_test_util.h"                  // nogncheck
 #include "ios/chrome/test/app/navigation_test_util.h"              // nogncheck
 #import "ios/chrome/test/app/settings_test_util.h"                 // nogncheck
+#import "ios/chrome/test/app/signin_test_util.h"                   // nogncheck
 #import "ios/chrome/test/app/static_html_view_test_util.h"         // nogncheck
 #import "ios/chrome/test/app/tab_test_util.h"                      // nogncheck
 #import "ios/web/public/test/earl_grey/js_test_util.h"             // nogncheck
@@ -421,11 +422,23 @@ id ExecuteJavaScript(NSString* javascript,
 }
 
 #pragma mark - Settings Utilities
+
 + (NSError*)setContentSettings:(ContentSetting)setting {
   chrome_test_util::SetContentSettingsBlockPopups(setting);
   return nil;
 }
 
+#pragma mark - Sign Utilities
+
++ (NSError*)signOutAndClearAccounts {
+  bool success = chrome_test_util::SignOutAndClearAccounts();
+  if (!success) {
+    return testing::NSErrorWithLocalizedDescription(
+        @"Real accounts couldn't be cleared.");
+  }
+  return nil;
+}
+
 @end
 
 #endif  // defined(CHROME_EARL_GREY_1)