perf hists browser: Update the browser.nr_entries after the timer

Previously the hist_browser dealt with a static tree of entries, now it
needs to update the nr_entries in the browser after the timer runs.

A better solution will come when moving using another thread for the
collapse_resort, etc, but for now this is ok.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-9eno2iq55sjr4iyo899buzaw@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>