Remove isMainThread() checks from ScriptForbiddenScope

r177457 added isMainThread() checks to ScriptForbiddenScope and regressed performance. This CL removes the checks by making sure that ScriptForbiddenScope is not used in non-main threads.

Review URL: https://codereview.chromium.org/426243004

git-svn-id: svn://svn.chromium.org/blink/trunk@179255 bbb929c8-8fbe-4397-9dbb-9b2b20218538