DevTools: weird behaviour of scroll bar is observed in "Frames" section

TBR=vsevik
BUG=340943

Review URL: https://codereview.chromium.org/132113007

git-svn-id: svn://svn.chromium.org/blink/trunk@166518 bbb929c8-8fbe-4397-9dbb-9b2b20218538