Exposed to the extras as extrasExports (on the builtins object), on
which they can put things that should be accessible from C++. Exposed
to C++ through the V8 API as v8::Context::GetExtrasExportsObject().

Adding a test (in test-api.cc) required adding a simple extra,
test-extra.js, which we build into the standalone builds.

R=yangguo@chromium.org, jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1128113006

Cr-Commit-Position: refs/heads/master@{#28317}