Simplify v8/Isolate teardown.

This implies that one better has a v8::V8::Initialize when v8::V8::Dispose is used.

BUG=359977
LOG=y
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/238353015

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20876 ce2b1a6d-e550-0410-aec6-3dcde31c8c00