Reenables PasswordManagerInteractiveTest.PromptForXHRWithoutOnSubmit

For ChromeOS with single-process-mash. I ran this test 100 times and saw
no flake.

BUG=905789
TEST=test only change
TBR=msw@chromium.org

Change-Id: I9fa491da4a20bfdbca8385295b1a9c20359df8f4
Reviewed-on: https://chromium-review.googlesource.com/c/1374693Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616079}

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

@@ -38,6 +38,3 @@
 
 # This test is flaky. https://crbug.com/897879
 -ExtensionApiTest.DisplayModeWindowIsInFullscreen
-
-# This test is flaky. https://crbug.com/905789
--PasswordManagerInteractiveTest.PromptForXHRWithoutOnSubmit