Use new platform TraceEvent API in blink.

Step 3 of migrating blink TraceEvent to use the new API.

BUG=trace-viewer:487

Review URL: https://codereview.chromium.org/29103006

git-svn-id: svn://svn.chromium.org/blink/trunk@160276 bbb929c8-8fbe-4397-9dbb-9b2b20218538