Move ScriptForbiddenScope from core/dom/ to platform/

Because Oilpan wants to use ScriptForbiddenScope in platform/heap/.

BUG=340522

Review URL: https://codereview.chromium.org/364173003

git-svn-id: svn://svn.chromium.org/blink/trunk@177569 bbb929c8-8fbe-4397-9dbb-9b2b20218538