[Sheriff] Disable flaky SafeBrowsing test

Variations of the test are flaking often on Mac10.15

TBR=timvolodine@chromium.org

Bug: 1096054
Change-Id: Ib3258a6f78bfa4e86d97089be180517d50ca8e2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2249782Reviewed-by: Ioana Pandele <ioanap@chromium.org>
Commit-Queue: Ioana Pandele <ioanap@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780281}

chrome/browser/safe_browsing/safe_browsing_blocking_page_test.cc

@@ -921,8 +921,15 @@ IN_PROC_BROWSER_TEST_P(SafeBrowsingBlockingPageBrowserTest, IframeProceed) {
             browser()->tab_strip_model()->GetActiveWebContents()->GetURL());
 }
 
+#if defined(OS_MACOSX)
+#define MAYBE_IframeOptInAndReportThreatDetails \
+  DISABLED_IframeOptInAndReportThreatDetails
+#else
+#define MAYBE_IframeOptInAndReportThreatDetails \
+  IframeOptInAndReportThreatDetails
+#endif
 IN_PROC_BROWSER_TEST_P(SafeBrowsingBlockingPageBrowserTest,
-                       IframeOptInAndReportThreatDetails) {
+                       MAYBE_IframeOptInAndReportThreatDetails) {
   // The extended reporting opt-in is presented in the interstitial for malware,
   // phishing, and UwS threats.
   const bool expect_threat_details =