bindings: Aligns ScriptWrappableBase to 4 byte alignment.

Because of a MSVC's bug, we need to explicitly specify the alignment requirement by using __declspec(align(4)).

See the comment in ScriptWrappable.h for details.

BUG=235436

Review URL: https://codereview.chromium.org/487723003

git-svn-id: svn://svn.chromium.org/blink/trunk@180556 bbb929c8-8fbe-4397-9dbb-9b2b20218538