Disable flaky DownloadContentTest.ForceDownloadMultipartRelatedPage

TBR=qinmin@chromium.org

Bug: 837104
Change-Id: I8f59a618815ce71de395af29795b891529a75ef4
Reviewed-on: https://chromium-review.googlesource.com/1029554Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553923}

content/browser/download/download_browsertest.cc

@@ -3376,7 +3376,8 @@ IN_PROC_BROWSER_TEST_F(DownloadContentTest, FetchErrorResponseBodyResumption) {
             std::string("header_value"));
 }
 
-IN_PROC_BROWSER_TEST_F(DownloadContentTest, ForceDownloadMultipartRelatedPage) {
+IN_PROC_BROWSER_TEST_F(DownloadContentTest,
+                       DISABLED_ForceDownloadMultipartRelatedPage) {
   // Force downloading the MHTML.
   DownloadTestContentBrowserClient new_client;
   new_client.set_allowed_rendering_mhtml_over_http(false);