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

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

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

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1287113010

Cr-Commit-Position: refs/heads/master@{#30270}