Fixes issues raised in CL https://codereview.chromium.org/2887193002.
That is:

1) Remove using mutex in Isolate::InitializeCounters().

2) Use counters_shared_.get() instead of counters_ (and hence, also
   remove field counters_).

BUG=v8:6361

Review-Url: https://codereview.chromium.org/2919953003
Cr-Commit-Position: refs/heads/master@{#45743}