Tweaks for scrollbar insets

Take 2 on insets.

BUG=333499
TEST=none
R=sadrul@chromium.org
TBR=sadrul@chromium.org

Review URL: https://codereview.chromium.org/148223015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247667 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247667