Gardening: Layout Test fast/html/imports/import-custom-element-in-grandchild.html is flaky

BUG=370814
TBR= morrita
NOTRY=true

Review URL: https://codereview.chromium.org/267373004

git-svn-id: svn://svn.chromium.org/blink/trunk@173503 bbb929c8-8fbe-4397-9dbb-9b2b20218538