Extensions: Add UMA to monitor whether a webview request has DWR rules to be evaluated.

extensions/browser/api/web_request/web_request_api.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "extensions/browser/api/web_request/web_request_api.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <memory>
@@ skipping 1981 matching lines @@
     BlockedRequest& blocked_request = blocked_requests_[request->identifier()];
     blocked_request.num_handlers_blocking++;
     blocked_request.request = request;
     blocked_request.is_incognito |= IsIncognitoBrowserContext(browser_context);
     blocked_request.blocking_time = base::Time::Now();
     blocked_request.original_response_headers = original_response_headers;
     blocked_request.extension_info_map = extension_info_map;
     return true;
   }
 
+  if (is_web_view_guest) {
+    const bool has_declarative_rules = !relevant_registries.empty();
+    UMA_HISTOGRAM_BOOLEAN(
+        "Extensions.DeclarativeWebRequest.WebViewRequestDeclarativeRules",

[lazyboy, 2017/05/05 17:46:45]

relevant_registries.empty() == true doesn't necessarily mean that webview has
applicable rules, it only means it might have applicable rules in the registry,
right? Because if deltas_created is false below, then it wouldn't have any rules
to apply.
I'd change the description/name in relevant places to reflect that.

Also I'd be clear about the motivation of this UMA in CL description.

[karandeepb, 2017/05/05 18:40:27]

Have removed the usage of the word applicable and changed it to clarify that the
request has rules that need to be evaluated. Have also updated the CL
description.

+        has_declarative_rules);
+  }
+
   base::Time start = base::Time::Now();
 
   bool deltas_created = false;
   for (const auto& it : relevant_registries) {
     WebRequestRulesRegistry* rules_registry = it.first;
     helpers::EventResponseDeltas result = rules_registry->CreateDeltas(
         extension_info_map,
         WebRequestData(request, request_stage, navigation_ui_data,
                        original_response_headers),
         it.second);
@@ skipping 422 matching lines @@
   // Since EventListeners are segmented by browser_context, check that
   // last, as it is exceedingly unlikely to be different.
   return extension_id == that.extension_id &&
          sub_event_name == that.sub_event_name &&
          web_view_instance_id == that.web_view_instance_id &&
          embedder_process_id == that.embedder_process_id &&
          browser_context == that.browser_context;
 }
 
 }  // namespace extensions