enable download protection on CrOS

chrome/browser/download/notification/download_notification_browsertest.cc

Index: chrome/browser/download/notification/download_notification_browsertest.cc
diff --git a/chrome/browser/download/notification/download_notification_browsertest.cc b/chrome/browser/download/notification/download_notification_browsertest.cc
index 6d08b3f26f9c220ce82c50cb8a3b482787034884..b6578d400f0577d2fc0a8b0b6c330bae6ff3407d 100644
--- a/chrome/browser/download/notification/download_notification_browsertest.cc
+++ b/chrome/browser/download/notification/download_notification_browsertest.cc
@@ -230,6 +230,13 @@ class TestChromeDownloadManagerDelegate : public ChromeDownloadManagerDelegate {
   // Return if  the download is opened.
   bool opened() const { return opened_; }
 
+ protected:
+  // Disable DownloadProtectionService in order to disable content checking.
+  safe_browsing::DownloadProtectionService* GetDownloadProtectionService()
+      override {
+    return nullptr;
+  }
+
  private:
   bool opened_;
 };