add a flag to enable/disable download interception on android

chrome/browser/android/intercept_download_resource_throttle.cc

Index: chrome/browser/android/intercept_download_resource_throttle.cc
diff --git a/chrome/browser/android/intercept_download_resource_throttle.cc b/chrome/browser/android/intercept_download_resource_throttle.cc
index 1592233f81895c4a8ad36a5e77f949d2378b0100..4f5662fa13f0afc3063a4d1087bd63476739a4bd 100644
--- a/chrome/browser/android/intercept_download_resource_throttle.cc
+++ b/chrome/browser/android/intercept_download_resource_throttle.cc
@@ -4,7 +4,9 @@
 
 #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"
@@ -41,6 +43,11 @@ void RecordInterceptFailureReasons(
 
 namespace chrome {
 
+// static
+bool InterceptDownloadResourceThrottle::IsDownloadInterceptionEnabled() {
+  return base::FeatureList::IsEnabled(chrome::android::kSystemDownloadManager);
+}
+
 InterceptDownloadResourceThrottle::InterceptDownloadResourceThrottle(
     net::URLRequest* request,
     int render_process_id,
@@ -64,6 +71,9 @@ const char* InterceptDownloadResourceThrottle::GetNameForLogging() const {
 }
 
 void InterceptDownloadResourceThrottle::ProcessDownloadRequest() {
+  if (!IsDownloadInterceptionEnabled())
+    return;
+
   if (request_->url_chain().empty()) {
     RecordInterceptFailureReasons(EMPTY_URL);
     return;