Move scrollend-event-fired-after-snap.html back to content shell

This test appears to be flaky after migrated to headless shell, even
though at a low rate. See flaky history [1] for details.

Move this test back to run with content shell.

[1] https://ci.chromium.org/ui/test/chromium/ninja%3A%2F%2F%3Aheadless_shell_wpt%2Fexternal%2Fwpt%2Fdom%2Fevents%2Fscrolling%2Fscrollend-event-fired-after-snap.html

Bug: 397486853
Change-Id: I4335f9b490f36f7a7e283b1edf0c46e81ca8b0fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6279071
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/main@{#1422056}