DevTools: Make timeline event timestamps zero-based

Make timeline event timestamps zero-based and provide starting time and
timestamp once for the entire timeline.

R=caseq@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/22478007

git-svn-id: svn://svn.chromium.org/blink/trunk@155778 bbb929c8-8fbe-4397-9dbb-9b2b20218538