Disable WebStateTest.MessageFromIFrame test.

TBR=eugenebut@chromium.org

Bug: 857129
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I407a6464e71296cc15eef43bf7ed2a8ca8181909
Reviewed-on: https://chromium-review.googlesource.com/1117300
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570882}

ios/web/web_state/web_state_unittest.mm

@@ -211,6 +211,7 @@ TEST_F(WebStateTest, MessageFromMainFrame) {
 
 // Tests that message sent from main frame triggers the ScriptCommandCallback
 // with |is_main_frame| = false.
+// TODO(crbug.com/857129): Re-enable this test.
 TEST_F(WebStateTest, MessageFromIFrame) {
   // Add a script command handler.
   __block bool message_received = false;