Normal trace events already go directly to base. This CL removes the remaining
trace events that were still indirected through blink::Platform. These were
just the half-finished inspector timeline trace events, which currently aren't
actually hooked up to anything.
This CL also removes the redunant "convertable to trace format" wrappers and
moves their one remaining use over to just using the version in base directly.
R=eseidel@google.com, ojan@chromium.org
Review URL: https://codereview.chromium.org/889823002
e962904c7a