Trace document.write/element.innerHTML

Turns out we were tracing async parse events
during main document parse, but not synchronous
parse events like innerHTML from javascript.

This just adds a simple trace event for
HTMLDocumentParser::insert which is the
synchronous parsing entry point.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/27781003

git-svn-id: svn://svn.chromium.org/blink/trunk@159929 bbb929c8-8fbe-4397-9dbb-9b2b20218538