ScriptPromise should cast the constructor parameter.

Currently, passing a non-Promise ScriptValue to the ScriptPromise constructor leads to crash.
Since we have Promise.cast, casting the value seems reasonable.

BUG=337700
R=abarth

Review URL: https://codereview.chromium.org/131413013

git-svn-id: svn://svn.chromium.org/blink/trunk@165733 bbb929c8-8fbe-4397-9dbb-9b2b20218538