Change WebKitPlatformSupportImpl::addTraceEvent() to new impl

Step 4 of migrating WebKit TraceEvent to use new API.

BUG=trace-viewer:487

Review URL: https://codereview.chromium.org/29303005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230520 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230520