TimelinePanel: fix for white horizontal bar in records view after initial show.

parent of recordsView had x-scrollbar and affected the size of child views.

Drive by fix: disable y-scrollbar of Memory counters sidebar view.

BUG=326121
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/143323002

git-svn-id: svn://svn.chromium.org/blink/trunk@165403 bbb929c8-8fbe-4397-9dbb-9b2b20218538