bindings: Refactors v8SetReturnValue.

Reduces generated code of v8SetReturnValue and toV8 for ScriptWrappable objects.

Will simplify more for non-ScriptWrappable objects in a separate CL.

BUG=235436

Review URL: https://codereview.chromium.org/646523004

git-svn-id: svn://svn.chromium.org/blink/trunk@184575 bbb929c8-8fbe-4397-9dbb-9b2b20218538