DevTools: speculative fix to consolev-viewport-selection.html flakiness

The flakiness dashboard reports that test "testScrollSelectionAwayDown"
from console-viewport-selection.html testsuite is flaky. The test
uses "forceItemToBeLast" viewport method which, for some reason, turned
out to be flaky. The patch gets rid of the method usage in this test.

BUG=390204
TBR=vsevik

Review URL: https://codereview.chromium.org/495583003

git-svn-id: svn://svn.chromium.org/blink/trunk@180834 bbb929c8-8fbe-4397-9dbb-9b2b20218538