Avoid crash on Promise::createPending

V8PromiseCustom::createPromise takes |creationContext| parameter.
Currently we pass toV8Context(context, world)->Global() for the parameter,
but toV8Context(context, ...) returns NULL when context is a document
without a Frame.
Changed to use a newly created object for the creation
context under such situation.

R=haraken
BUG=313143

Review URL: https://codereview.chromium.org/51623004

git-svn-id: svn://svn.chromium.org/blink/trunk@162190 bbb929c8-8fbe-4397-9dbb-9b2b20218538