The code for UMA stats (in counters.h) is not thread safe, and can
lead to using pointers with uninitialized values.

Therefore, this CL turns them off when compiling asynchronously.

It also turns back on several UMA stats that were previously turned
off, but no longer need to because the code now knows if it is
running synchronously.

BUG=v8:6361

Review-Url: https://codereview.chromium.org/2864583004
Cr-Commit-Position: refs/heads/master@{#45168}