This is a fix to https://codereview.chromium.org/2929853003 that got
reverted. The DCHECK checked to see that it was not in a background
thread. While this is a property we want for v8, it is also used
by blink, and blink violates this property.

Therefore, this CL removes the DCHECK for now.

BUG=v8:6361

Review-Url: https://codereview.chromium.org/2961443002
Cr-Commit-Position: refs/heads/master@{#46190}