Pass isolate to ScriptPromise and ScriptString constructors

Pass isolate to ScriptPromise and ScriptString constructors to avoid calls
to v8::Isolate::GetCurrent(). Also add an m_isolate member to ScriptValue
to avoid avoid calls to v8::Isolate::GetCurrent() and simplify the API.

R=haraken
BUG=263412

Review URL: https://chromiumcodereview.appspot.com/23513066

git-svn-id: svn://svn.chromium.org/blink/trunk@157950 bbb929c8-8fbe-4397-9dbb-9b2b20218538