Revert "Forward declaration for Isolate / Platform in libplatform.h." and "Fixes d8 on windows following.".

This reverts commit 0f57ce8f.
and 3245b8cb for breaking Chromium
compile.

TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/743533003

Cr-Commit-Position: refs/heads/master@{#25411}