add a flag to enable/disable download interception on android

chrome/browser/android/chrome_feature_list.cc

Index: chrome/browser/android/chrome_feature_list.cc
diff --git a/chrome/browser/android/chrome_feature_list.cc b/chrome/browser/android/chrome_feature_list.cc
index 8b44469b7a37e05546fa58c7666eef0a6fce1ec9..91809b751d9fbd80aa84122d316fb7738c610fc4 100644
--- a/chrome/browser/android/chrome_feature_list.cc
+++ b/chrome/browser/android/chrome_feature_list.cc
@@ -38,6 +38,10 @@ const base::Feature kPhysicalWebFeature {
   "PhysicalWeb", base::FEATURE_DISABLED_BY_DEFAULT
 };
 
+const base::Feature kSystemDownloadManager {
+  "SystemDownloadManager", base::FEATURE_ENABLED_BY_DEFAULT
+};
+
 static jboolean IsEnabled(JNIEnv* env,
                           const JavaParamRef<jclass>& clazz,
                           const JavaParamRef<jstring>& jfeature_name) {