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

Due to the specificity of viewport, ViewportControl.forceScrollItemToBeLast(X)
scrolls viewport so that the element number X will be last, but with precision
of a couple of pixels. Given this, viewport might render additional elements
to cover the visible area.

That said, dumping the last rendered element index was a bad idea which
resulted in flakiness.

This patch eliminates any dumping of last rendered element, substituting it
with a first element index and a flag whether sufficient amount of elements
was rendered to cover viewport.

BUG=390204
TBR=vsevik

Review URL: https://codereview.chromium.org/393853002

git-svn-id: svn://svn.chromium.org/blink/trunk@178204 bbb929c8-8fbe-4397-9dbb-9b2b20218538