TimelinePanel: REGRESSION fix white line at the bottom of memory view when dock/undock

simple fix for merging into M-36
BUG=364448
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/242713003

git-svn-id: svn://svn.chromium.org/blink/trunk@171929 bbb929c8-8fbe-4397-9dbb-9b2b20218538