[worklets] Implement isJSExecutionIsForbidden call for WorkletGlobalScope.

This method is currently unreachable for worklets (no events).

Additionally moves isClosing method.

BUG=567358

Review-Url: https://codereview.chromium.org/2223623002
Cr-Commit-Position: refs/heads/master@{#410343}
3 files changed