[iOS] Disable a fullscreen egtest on devices.

FullscreenTestCase/testShowHeaderWhenChildTabCloses is flaky on
devices.

Bug: 812664
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I7e632efa8823e253264c0d20012ac80c628562e7
Reviewed-on: https://chromium-review.googlesource.com/922261Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537102}

ios/chrome/browser/ui/fullscreen/fullscreen_egtest.mm

@@ -247,6 +247,11 @@ void AssertURLIs(const GURL& expectedURL) {
 // Test to make sure the header is shown when a Tab opened by the current Tab is
 // closed even if the toolbar was not present previously.
 - (void)testShowHeaderWhenChildTabCloses {
+// TODO(crbug.com/812664): Re-enable this test on devices.
+#if !TARGET_IPHONE_SIMULATOR
+  EARL_GREY_TEST_DISABLED(@"Test disabled on device.");
+#endif
+
   std::map<GURL, std::string> responses;
   const GURL URL = web::test::HttpServer::MakeUrl("http://origin");
   const GURL destinationURL =