1) I moved the implementations of the wrapper functions into a new cc
file so that I can use these wrapper functions in tests.

2) I made a generic test for all tests in
test-run-calls-to-external-references.cc. In the new test we only
compare the result of a function call through an external reference with
the result of a direct function call. This is sufficient because we only
want to test function calls through external references work here.
The implementation of these functions are tested somewhere else.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1853123002

Cr-Commit-Position: refs/heads/master@{#35289}