This commit ensures that the d8 shared library build uses the same logic as
the standard static build by exporting relevant functions and classes.

BUG=chromium:646337

Committed: https://crrev.com/2c10ca8086a4d595ecf9aa843d2031b068470d65
Review-Url: https://codereview.chromium.org/2342563002
Cr-Original-Commit-Position: refs/heads/master@{#39503}
Cr-Commit-Position: refs/heads/master@{#39547}