Exercise stress test against the virtual CPU ID, if applicable.

* The Unsafe form is acceptable, since we've checked IsFast at the start of the
  test.  We will always have an initialized CPU ID.
* Since Grow/Shrink rely on CompareAndSwap against the current CPU ID, we
  need to be in the same CPU ID space.

PiperOrigin-RevId: 318540571
Change-Id: If7e5829c35bb31f3c47093b1ad65eabefaa4039f

Author: ckennelly

tcmalloc/internal/percpu_tcmalloc_test.cc

@@ -575,13 +575,14 @@ static void StressThread(size_t thread_id, TcmallocSlab* slab,
         }
       }
       if (n != 0) {
-        size_t res = slab->Grow(GetCurrentCpu(), cl, n, kStressCapacity);
+        size_t res =
+            slab->Grow(GetCurrentVirtualCpuUnsafe(), cl, n, kStressCapacity);
         EXPECT_LE(res, n);
         capacity->fetch_add(n - res);
       }
     } else if (what < 60) {
       size_t n =
-          slab->Shrink(GetCurrentCpu(), cl,
+          slab->Shrink(GetCurrentVirtualCpuUnsafe(), cl,
                        absl::Uniform<int32_t>(rnd, 0, kStressCapacity) + 1);
       capacity->fetch_add(n);
     } else if (what < 70) {