Fix lint error in TimelinePanel.js due to 'energy' variable.

This error was introduced in: https://codereview.chromium.org/220963002

BUG=

Review URL: https://codereview.chromium.org/236083009

git-svn-id: svn://svn.chromium.org/blink/trunk@171434 bbb929c8-8fbe-4397-9dbb-9b2b20218538