Make isolate.h usable without objects-inl.h header.

Make isolate.h usable without objects-inl.h header.

This CL us a pure refactoring that makes an empty compilation unit
including just "isolate.h" or "contexts.h" but not "objects-inl.h"
compile without warnings or errors. This is needed to further reduce
the header dependency tangle.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1322883002

Cr-Commit-Position: refs/heads/master@{#30500}