DevTools: Attempt to make http/tests/inspector/elements/styles/selector-line-sourcemap-header-deprecated.html non-flaky

R=pfeldman, vsevik
NOTRY=true

Review URL: https://codereview.chromium.org/221933005

git-svn-id: svn://svn.chromium.org/blink/trunk@170652 bbb929c8-8fbe-4397-9dbb-9b2b20218538