Adds autoscoll on/off option for log output in Gwt Hosted mode window.

Fixes issues: 5224

Change-Id: I2dfa2b70d264804c4b0a52811923ca09fd4d5969
Review-Link: https://gwt-review.googlesource.com/#/c/1220/

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11381 8db76d5a-ed1c-0410-87a9-c151d255dfc7