Mark flaky test cross-origin-iframe.sub.html

The flakes are most notable on Mac but occur on multiple platforms.

TBR=rajendrant@chromium.org
BUG=1004547

Change-Id: Ia17fb182e574244b1f792729fa493f31a3aed529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807307Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697589}

third_party/blink/web_tests/TestExpectations

@@ -248,6 +248,8 @@ crbug.com/980969 [ Mac ] http/tests/input/discard-events-to-unstable-iframe.html
 crbug.com/1002049 external/wpt/css/mediaqueries/aspect-ratio-005.html [ Failure ]
 crbug.com/1002049 external/wpt/css/mediaqueries/aspect-ratio-006.html [ Failure ]
 
+crbug.com/1004547 external/wpt/intersection-observer/cross-origin-iframe.sub.html [ Pass Failure ]
+
 # Display locking failures
 crbug.com/955533 wpt_internal/display-lock/sizing/overflow-auto-with-overflow.html [ Failure ]