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

Committed: https://crrev.com/8a8867d341b66eac303eb8baa6239a2f43181c79
Cr-Commit-Position: refs/heads/master@{#30270}

[Yang, 2015-08-20 13:00:05]

On 2015/08/20 12:36:35, Michael Starzinger wrote:

lgtm.

[Michael Starzinger, 2015-08-20 13:04:36]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-08-20 13:04:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1287113010/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1287113010/1

[commit-bot: I haz the power, 2015-08-20 13:06:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-20 13:06:18]

Patchset 1 (id:??) landed as
https://crrev.com/8a8867d341b66eac303eb8baa6239a2f43181c79
Cr-Commit-Position: refs/heads/master@{#30270}