DevTools: Make TimelineEvent records typed in backend.

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

Review URL: https://codereview.chromium.org/113353004

git-svn-id: svn://svn.chromium.org/blink/trunk@164332 bbb929c8-8fbe-4397-9dbb-9b2b20218538