Fix MultiProfileDownloadNotificationTest.DownloadMultipleFiles flakiness

This test occasionally crashes with the network service enabled because
a request hits the Interceptor after its been deleted.

Change-Id: Ib09e8c6cdf1208ced3297af37e2261c880bae0f1
Reviewed-on: https://chromium-review.googlesource.com/c/1478307Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633809}

chrome/browser/download/notification/download_notification_browsertest.cc

@@ -286,7 +286,11 @@ class DownloadNotificationTestBase : public InProcessBrowserTest {
     }
   }
 
-  void TearDownOnMainThread() override { interceptor_.reset(); }
+  void TearDownOnMainThread() override {
+    // Make sure any pending requests have finished.
+    base::RunLoop().RunUntilIdle();
+    interceptor_.reset();
+  }
 
  protected:
   content::DownloadManager* GetDownloadManager(Browser* browser) {