Disable flaky test CrossOriginIsolatedSiteInstance_MainFrame on Win

The test has been flaky (score > 1400):
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=content_browsertests%20(with%20patch)&tests=All%2FCrossOriginOpenerPolicyBrowserTest.CrossOriginIsolatedSiteInstance_MainFrame

https://analysis.chromium.org/p/chromium/flake-portal/flakes/occurrences?key=ag9zfmZpbmRpdC1mb3ItbWVydQsSBUZsYWtlImpjaHJvbWl1bUBjb250ZW50X2Jyb3dzZXJ0ZXN0c0BDcm9zc09yaWdpbk9wZW5lclBvbGljeUJyb3dzZXJUZXN0LkNyb3NzT3JpZ2luSXNvbGF0ZWRTaXRlSW5zdGFuY2VfTWFpbkZyYW1lDA

TBR=ahemery@chromium.org
Bug: 1125998

Change-Id: I991e18b0d74e08db3fe0360c6f43b2a284921be1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559006
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830763}

content/browser/cross_origin_opener_policy_browsertest.cc

@@ -4,6 +4,7 @@
 
 #include "base/test/bind.h"
 #include "base/test/scoped_feature_list.h"
+#include "build/build_config.h"
 #include "components/network_session_configurator/common/network_switches.h"
 #include "content/browser/renderer_host/navigation_request.h"
 #include "content/browser/renderer_host/render_frame_host_impl.h"
@@ -1970,8 +1971,16 @@ IN_PROC_BROWSER_TEST_P(VirtualBrowsingContextGroupTest,
   EXPECT_NE(group_4, group_1);
 }
 
+// This test is flaky on Win: https://crbug.com/1125998.
+#if defined(OS_WIN)
+#define MAYBE_CrossOriginIsolatedSiteInstance_MainFrame \
+  DISABLED_CrossOriginIsolatedSiteInstance_MainFrame
+#else
+#define MAYBE_CrossOriginIsolatedSiteInstance_MainFrame \
+  CrossOriginIsolatedSiteInstance_MainFrame
+#endif
 IN_PROC_BROWSER_TEST_P(CrossOriginOpenerPolicyBrowserTest,
-                       CrossOriginIsolatedSiteInstance_MainFrame) {
+                       MAYBE_CrossOriginIsolatedSiteInstance_MainFrame) {
   GURL isolated_page(
       https_server()->GetURL("a.com",
                              "/set-header?"