Disable ExtensionApiTest.DisplayModeWindowIsInFullscreen on Mash

Bug: 898169
Change-Id: I2cd883b81a111242691b038ff512652e5faa2a94
Reviewed-on: https://chromium-review.googlesource.com/c/1297055Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602073}

testing/buildbot/filters/chromeos.single_process_mash.interactive_ui_tests.filter

@@ -101,4 +101,4 @@
 -OobeInteractiveUITest.SimpleEndToEnd
 
 # This test is flaky. https://crbug.com/897879
--ExtensionApiTest.DisplayModeWindowIsInFull
+-ExtensionApiTest.DisplayModeWindowIsInFullscreen