[Sheriff] Disable flaky PrerenderBrowserTest.PrerenderSafeBrowsingIframe

Example failures:
https://ci.chromium.org/p/chromium/builders/ci/Win%207%20Tests%20x64%20(1)/64102
https://ci.chromium.org/p/chromium/builders/ci/Linux%20ChromiumOS%20MSan%20Tests/17727

TBR=tbansal@chromium.org,darin@chromium.org

Bug: 1050143
Change-Id: I5587e526b060621454d91d872387b2baf5b3270b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050748Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740486}

chrome/browser/prerender/prerender_browsertest.cc

@@ -1528,8 +1528,10 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderSafeBrowsingSubresource) {
   PrerenderTestURL(replacement_path, FINAL_STATUS_SAFE_BROWSING, 0);
 }
 
+// Flaky on multiple platforms, http://crbug.com/1050143
 // Ensures that we do not prerender pages which have a malware iframe.
-IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderSafeBrowsingIframe) {
+IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
+                       DISABLED_PrerenderSafeBrowsingIframe) {
   GURL iframe_url = embedded_test_server()->GetURL(
       "/prerender/prerender_embedded_content.html");
   GetFakeSafeBrowsingDatabaseManager()->SetThreatTypeForUrl(