DevTools: fix inspector/console/console-viewport-selection.html flakiness

It turns out that flakes happen only in tests that use
ViewportControl.forceScrollItemToBeLast method. This patch starts using
ViewportControl.forceScrollItemToBeFirst instead.

BUG=390204
TBR=vsevik

Review URL: https://codereview.chromium.org/399753003

git-svn-id: svn://svn.chromium.org/blink/trunk@178367 bbb929c8-8fbe-4397-9dbb-9b2b20218538