When ParseOnBackground is done, we should construct a trace event and dump all
background parsing runtime statistics as argument. In tracing we don't want to
merge it back to to main thread, instead we show the trace event together with
background parsing trace event on script streamer thread track in Trace Viewer.

To perf sheriff: A series of counters ParseBackgroundxxx and
PreparseBackgroundxxx will be taken into account in this patch, thus runtime
statistics graph will increase after this patch gets landed.

BUG=v8:5089

Review-Url: https://codereview.chromium.org/2559403002
Cr-Commit-Position: refs/heads/master@{#41658}