Devtools: revert "DevTools: Make timeline event timestamps zero-based".
(It was done with no good reason + prevents us from implementing console.timeline properly)

TBR=yurys

Review URL: https://chromiumcodereview.appspot.com/23466007

git-svn-id: svn://svn.chromium.org/blink/trunk@157310 bbb929c8-8fbe-4397-9dbb-9b2b20218538