Marking http/tests/htmlimports/csp-import-block-but-nonce-nested.html as Slow

Execution time of layout test
http/tests/htmlimports/csp-import-block-but-nonce-nested.html
is flaky on windows.

BUG=393573
TBR=falken@chromium.org

Review URL: https://codereview.chromium.org/388283002

git-svn-id: svn://svn.chromium.org/blink/trunk@178061 bbb929c8-8fbe-4397-9dbb-9b2b20218538