Mark fast/css/css-selector-deeply-nested.html as not flaky

It stopped crashing with the update of V8 to version 3.28.21:
https://codereview.chromium.org/381383003

BUG=392853
NOTRY=true

Review URL: https://codereview.chromium.org/532283002

git-svn-id: svn://svn.chromium.org/blink/trunk@181397 bbb929c8-8fbe-4397-9dbb-9b2b20218538