[Extensions] Pass the runtime_variable ownership forward

We can avoid a few copies if we use the shared_ptr + .Pass() idiom.
[Extensions]: Expose the extension path to the extension

There is an use case, specially with pycrosswalk, where you can have
multiple instances of the same extension loaded using symbolic links.

The extension needs to know about itself (the path to be more precise)
to figure out what python script it should load.
Merge pull request #2121 from tmpsantos/extension_path

[Extensions]: Expose the extension path to the extension