DevTools: [Console] fix console-viewport-selection.html test flakiness

This patch fixes console-viewport-selection.html test flakiness.

BUG=390204
TBR=vsevik

Review URL: https://codereview.chromium.org/367353002

git-svn-id: svn://svn.chromium.org/blink/trunk@177475 bbb929c8-8fbe-4397-9dbb-9b2b20218538