DevTools: use platform/TraceEvent.h types in TimelineTraceEventProcessor

We used to have several types matching those in chromium defined locally
in TimelineTraceEventProcessor since WebKit times. Now that TraceEvent.h
is in platform we can switch over to types defined there.

BUG=

Review URL: https://codereview.chromium.org/27482003

git-svn-id: svn://svn.chromium.org/blink/trunk@160072 bbb929c8-8fbe-4397-9dbb-9b2b20218538