no member named 'CreateDefaultPlatform' in namespace 'v8::platform'

@stephenfarrar, CreateDefaultPlatform was [deprecated](https://github.com/v8/v8/commit/bc3d7298809cdf990aea2d0755f8263392e1ba7e) and [removed](https://github.com/v8/v8/commit/cbd8f4269f91e8ade38876092b433bda9ddb89a9)

So, now build of gjstest with the latest v8 fails.

