add a flag to enable/disable download interception on android

chrome/browser/android/intercept_download_resource_throttle.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 "chrome/browser/android/intercept_download_resource_throttle.h"
 
+#include "base/feature_list.h"
 #include "base/metrics/histogram_macros.h"
+#include "chrome/browser/android/chrome_feature_list.h"
 #include "components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h"
 #include "content/public/browser/android/download_controller_android.h"
 #include "content/public/browser/resource_controller.h"
 #include "net/http/http_request_headers.h"
 #include "net/http/http_response_headers.h"
 #include "net/url_request/url_request.h"
 
 namespace {
 
 // UMA histogram for tracking reasons that chrome fails to intercept the
@@ skipping 16 matching lines @@
     MobileDownloadInterceptFailureReason reason) {
   UMA_HISTOGRAM_ENUMERATION("MobileDownload.InterceptFailureReason",
                             reason,
                             FAILURE_REASON_SIZE);
 }
 
 }  // namespace
 
 namespace chrome {
 
+// static
+bool InterceptDownloadResourceThrottle::IsDownloadInterceptionEnabled() {
+  return base::FeatureList::IsEnabled(chrome::android::kSystemDownloadManager);
+}
+
 InterceptDownloadResourceThrottle::InterceptDownloadResourceThrottle(
     net::URLRequest* request,
     int render_process_id,
     int render_view_id,
     int request_id)
     : request_(request),
       render_process_id_(render_process_id),
       render_view_id_(render_view_id),
       request_id_(request_id) {
 }
 
 InterceptDownloadResourceThrottle::~InterceptDownloadResourceThrottle() {
 }
 
 void InterceptDownloadResourceThrottle::WillProcessResponse(bool* defer) {
   ProcessDownloadRequest();
 }
 
 const char* InterceptDownloadResourceThrottle::GetNameForLogging() const {
   return "InterceptDownloadResourceThrottle";
 }
 
 void InterceptDownloadResourceThrottle::ProcessDownloadRequest() {
+  if (!IsDownloadInterceptionEnabled())
+    return;
+
   if (request_->url_chain().empty()) {
     RecordInterceptFailureReasons(EMPTY_URL);
     return;
   }
 
   GURL url = request_->url_chain().back();
   if (!url.SchemeIsHTTPOrHTTPS()) {
     RecordInterceptFailureReasons(NON_HTTP_OR_HTTPS);
     return;
   }
@@ skipping 31 matching lines @@
     return;
   }
 
   content::DownloadControllerAndroid::Get()->CreateGETDownload(
       render_process_id_, render_view_id_, request_id_);
   controller()->Cancel();
   RecordInterceptFailureReasons(NO_FAILURE);
 }
 
 }  // namespace chrome