GitLab

We used to have trouble when the max scroll offset was zero. With this
cl we detect this case and set a reasonable scroll thumb offset.

BUG=None

Review URL: https://codereview.chromium.org/409193002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284773 0039d316-1c4b-4281-b951-d872f2087c98