Removing "on" prefix for chromeIdentityServiceWillBeDestroyed

Conflict between crrev.com/c/559001 and crrev.com/c/559101

Bug: 661794
Change-Id: Ia7601ba8f334adad8a33fb1cbd744fb050382856
Reviewed-on: https://chromium-review.googlesource.com/574236
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487429}

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

@@ -327,7 +327,7 @@ void RecordSigninNewAccountUserActionForAccessPoint(
   }
 }
 
-- (void)onChromeIdentityServiceWillBeDestroyed {
+- (void)chromeIdentityServiceWillBeDestroyed {
   _identityServiceObserver.reset();
 }