Destruct TraceEvent temporary objects before posting cross-thread tasks

TraceEvent temporary objects are destructed after cross-thread task posting,
which causes wrong reference count.
This change makes TraceEvent temporary objects to be destructed strictly
before crossing threads.

BUG=394706

Review URL: https://codereview.chromium.org/399893002

git-svn-id: svn://svn.chromium.org/blink/trunk@182298 bbb929c8-8fbe-4397-9dbb-9b2b20218538