DevTools: Attempt to make inspector/elements/edit-dom-actions-shadow.html non-flaky

R=vsevik
BUG=355459
NOTRY=true

Review URL: https://codereview.chromium.org/199633018

git-svn-id: svn://svn.chromium.org/blink/trunk@170649 bbb929c8-8fbe-4397-9dbb-9b2b20218538