Gardening: http/tests/inspector/elements/styles/selector-line-sourcemap-header-deprecated.html flaky

BUG=353179
TEST=
TBR=zmo@chromium.org

Review URL: https://codereview.chromium.org/201723006

git-svn-id: svn://svn.chromium.org/blink/trunk@169367 bbb929c8-8fbe-4397-9dbb-9b2b20218538