src: add InitializeV8Platform function

This commit adds an InitializeV8Platform function which calls
v8_platform's Initialize to create the NodePlatform and also set the
structs members.

When running cctests this functions was not being called (it is called
from the Start function but that function is not called by the test
fixture.

The motivation for adding this is that I'm guessing that embedders
would might need the ability to do the same thing.

Refs: https://github.com/nodejs/node-v8/issues/69

PR-URL: https://github.com/nodejs/node/pull/21983


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>