Add new TraceEvent API in WebKit::Platform.

The new Trace API supports updating an existing TraceEvent's duration
by the TraceEventHandle returned from addTraceEvent() (which is
temporarily named addTraceEventNew).

Will take the following steps to smoothly migrate to the new API:
1. This CL to add the new APIs (new addTraceEvent is named
   addTraceEventNew temporarily);
2. A chromium CL to implement the new APIs;
3. A blink change to use the new APIs and remove the old
   Platform::addTraceEvent();
4. A chromium CL to change the old addTraceEvent to new API and
   implementation;
5. A blink change to rename addTraceEventNew() to addTraceEvent;
6. A chromium CL to remove the temporary addTraceEventNew();

BUG=trace-viewer:487

Review URL: https://codereview.chromium.org/29693007

git-svn-id: svn://svn.chromium.org/blink/trunk@160043 bbb929c8-8fbe-4397-9dbb-9b2b20218538