The trace-events will have a high overhead when turned on, but they are in a disabled-by-default category.

As long as the off overhead is negligible, this CL allows us to understand the behavior of V8 rather than its performance at the moment.

The original CL was failing the TSAN builder, the variable in question was intended to be accessed quickly with no guarantee.
Switched to using an Atomic variable with no barrier read/write.

BUG=v8:5089

patch from issue 2052523002 at patchset 100001 (http://crrev.com/2052523002#ps100001)

Review-Url: https://codereview.chromium.org/2063853002
Cr-Commit-Position: refs/heads/master@{#36973}