Update comment on SigninManagerAndroid to refer to current bug

Bug: 987965
Change-Id: I29b50b988983b2ec8ef976370a576af426dedbbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720833Reviewed-by: Lowell Manners <lowell@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681310}

chrome/browser/android/signin/signin_manager_android.cc

@@ -77,8 +77,9 @@ void SigninManagerAndroid::OnSignInCompleted(
     const JavaParamRef<jstring>& username) {
   DVLOG(1) << "SigninManagerAndroid::OnSignInCompleted";
 
-  // TODO(crbug.com/889902): Migrate to IdentityManager once there's an
-  // API mapping for SigninManager::SignIn().
+  // TODO(crbug.com/987965): Migrate away from this direct usage of
+  // PrimaryAccountManager and eliminate this class needing to know about
+  // PrimaryAccountManager.
   identity_manager_->GetPrimaryAccountManager()->SignIn(
       base::android::ConvertJavaStringToUTF8(env, username));
 }