Devtools gardening: mark console-viewport-selection.html as flaky

BUG=390204
TBR=lushnikov

Review URL: https://codereview.chromium.org/360753002

git-svn-id: svn://svn.chromium.org/blink/trunk@177237 bbb929c8-8fbe-4397-9dbb-9b2b20218538