Updates of current-function and cumulative stats are done entirely in TLS.  At the end, cumulative stats are merged across all threads' TLS into the global cumulative stats.

Printing of cumulative stats after every function is removed, since there's very little value from that.  It was probably done in the first place just to give partial cumulative information in the face of crashes or assertion failures.

BUG= none
R=jfb@chromium.org

Review URL: https://codereview.chromium.org/887213002