By moving StatsTable from class Isolate to class Counters, it make the
class StatsTable thead safe. This is needed because these two classes
call each other, and for background compilation, instances of the
Counters class can persist longer that the corresponding Isolate it
came from.

It also removes unnecessary hops to the the Isolate, and checks if the
StatsTable has been created, for these communications.

BUG=v8:6361

Review-Url: https://codereview.chromium.org/2906063002
Cr-Commit-Position: refs/heads/master@{#45576}