[sheriff] Disable FullscreenElementInMultipleSubframes

Disable
SitePerProcessInteractiveBrowserTest.FullscreenElementInMultipleSubframes,
it's flaky on all platforms.

TBR=alexmos@chromium.org

Bug: 756338

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I6887ea1ef98138f877ec1174208f616f7f670e64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225356Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773737}

chrome/browser/site_isolation/site_per_process_interactive_browsertest.cc

@@ -974,15 +974,9 @@ IN_PROC_BROWSER_TEST_F(SitePerProcessInteractiveBrowserTest,
 // The test also exits fullscreen by simulating pressing ESC rather than using
 // document.webkitExitFullscreen(), which tests the browser-initiated
 // fullscreen exit path.
-#if defined(OS_CHROMEOS) || defined(OS_MACOSX)
-#define MAYBE_FullscreenElementInMultipleSubframes \
-  DISABLED_FullscreenElementInMultipleSubframes
-#else
-#define MAYBE_FullscreenElementInMultipleSubframes \
-  FullscreenElementInMultipleSubframes
-#endif
+// TODO(crbug.com/756338): flaky on all platforms.
 IN_PROC_BROWSER_TEST_F(SitePerProcessInteractiveBrowserTest,
-                       MAYBE_FullscreenElementInMultipleSubframes) {
+                       DISABLED_FullscreenElementInMultipleSubframes) {
   // Allow fullscreen in all iframes descending to |c_middle|.
   GURL main_url(embedded_test_server()->GetURL(
       "a.com",