DevTools: fix console to tail messages on devtools with zoomfactor != 1

This patch fixes method Element.isScrolledToBottom. The reason for this
fix is that both clientHeight and scrollHeight are rounded to integer
values.

BUG=349399

Review URL: https://codereview.chromium.org/180733003

git-svn-id: svn://svn.chromium.org/blink/trunk@168643 bbb929c8-8fbe-4397-9dbb-9b2b20218538