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

TBR=dominicc@chromium.org, dglazkov@chromium.org
BUG=365956

Review URL: https://codereview.chromium.org/273513002

git-svn-id: svn://svn.chromium.org/blink/trunk@173456 bbb929c8-8fbe-4397-9dbb-9b2b20218538