DevTools: FlameChart follow-up fix for r158718. Do not show EntryInfo if there is no entry under cursor.

BUG=none
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/25727002

git-svn-id: svn://svn.chromium.org/blink/trunk@158722 bbb929c8-8fbe-4397-9dbb-9b2b20218538