Fix console auto-scroll for non-default zoom factor

Always try to scroll maximally upwards when autoscrolling console. No need to substract clientHeight from the scrollHeight to get precise maximall scrollable height, Blink will adjust it automatically to the max allowed value.

BUG=161646

Review URL: https://codereview.chromium.org/149633004

git-svn-id: svn://svn.chromium.org/blink/trunk@166712 bbb929c8-8fbe-4397-9dbb-9b2b20218538