about:sync-internals: Use RequestStop(CLEAR_DATA) instead of KEEP_DATA

CLEAR_DATA is what's typically used in practice, and KEEP_DATA is a
weird concept anyway.

Bug: 860482
Change-Id: Ia3d15463a03f947db19d08ebfd035299ee566eaf
Reviewed-on: https://chromium-review.googlesource.com/1146572Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577142}

chrome/browser/ui/webui/sync_internals_message_handler.cc

@@ -292,7 +292,7 @@ void SyncInternalsMessageHandler::HandleRequestStop(
   if (!service)
     return;
 
-  service->RequestStop(SyncService::KEEP_DATA);
+  service->RequestStop(SyncService::CLEAR_DATA);
 }
 
 void SyncInternalsMessageHandler::HandleTriggerRefresh(