Instead of four different constructors, we actually just need one. You
either pass a Counters*, or we will get it from the isolate (which is
only allowed to happen on the main thread).
This change makes refactoring this data structure for the baseline
compiler much easier.

R=mtrofin@chromium.org
CC=kschimpf@chromium.org

Bug: v8:6600
Change-Id: I56fb47005861dd4a203373776901930a02e09deb
Reviewed-on: https://chromium-review.googlesource.com/657979Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47964}