Convert timestamp to milliseconds when creating TracingModel.Event

Timeline uses time in milliseconds while trace event use microseconds. Do the conversion earlier.

BUG=361045

Review URL: https://codereview.chromium.org/318093002

git-svn-id: svn://svn.chromium.org/blink/trunk@175746 bbb929c8-8fbe-4397-9dbb-9b2b20218538