DevTools: Timeline: show correct information in cpu bar popover

startTime and endTime used to be the numbers and now they are functions.
We missed that because we had wrong type annotation.

BUG=390507

Review URL: https://codereview.chromium.org/363263002

git-svn-id: svn://svn.chromium.org/blink/trunk@177465 bbb929c8-8fbe-4397-9dbb-9b2b20218538