Add RELEASE_ASSERT for ensuring that we have no empty context to V8WrapperInstantiationScope.

BUG=369451

Review URL: https://codereview.chromium.org/315043007

git-svn-id: svn://svn.chromium.org/blink/trunk@176002 bbb929c8-8fbe-4397-9dbb-9b2b20218538