TRACE_EVENT header from Chromium/WebKit, with support functions to allow
plumbing back to an embedder.

Also adds some TRACE_EVENTs to startup flow.

Author: scottmg@google.com
R=apatrick@google.com, shannonwoods@google.com
BUG=crbug.com/270179

Review URL: https://codereview.appspot.com/12699047