Fix flakiness on import-custom-element-in-grandchild.html

BUG=370814
TBR=dominicc@chromium.org,dglazkov@chromium.org

Review URL: https://codereview.chromium.org/273523006

git-svn-id: svn://svn.chromium.org/blink/trunk@173558 bbb929c8-8fbe-4397-9dbb-9b2b20218538