[search_engines] Protect DSE from removal by the SearchEngineTabHelper

Two weeks ago we added a CHECK to verify in production our assumption
that the default search provider was never Remove()-ed.

This turned out to be violated in SearchEngineTabHelper.

This CL fixes that violation. We don't need to call Remove() from that
tab helper anyways, since a call to Add() will remove duplicates if
possible, and will preserve the DSE.

Bug: 1166372
Change-Id: Ia3d145ddc457d901fbb81c94487cee28b2792cca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637957Reviewed-by: Orin Jaworski <orinj@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844883}

chrome/browser/ui/search_engines/search_engine_tab_helper.cc

@@ -204,14 +204,11 @@ void SearchEngineTabHelper::GenerateKeywordIfNecessary(
   if (!url_service->CanAddAutogeneratedKeyword(keyword, url, &current_url))
     return;
 
-  if (current_url) {
-    if (current_url->originating_url().is_valid()) {
+  if (current_url && current_url->originating_url().is_valid()) {
     // The existing keyword was generated from an OpenSearch description
     // document, don't regenerate.
     return;
   }
-    url_service->Remove(current_url);
-  }
 
   TemplateURLData data;
   data.SetShortName(keyword);
@@ -233,6 +230,9 @@ void SearchEngineTabHelper::GenerateKeywordIfNecessary(
   }
   data.safe_for_autoreplace = true;
   data.input_encodings.push_back(handle->GetSearchableFormEncoding());
+
+  // This Add() call may displace the previously auto-generated TemplateURL,
+  // and that's expected. But it will never displace the default search engine.
   url_service->Add(std::make_unique<TemplateURL>(data));
 }