Make some foo.h headers usable without foo-inl.h header.

Make some foo.h headers usable without foo-inl.h header.

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

This realizes above state for "spaces.h" and "mark-compact.h".

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1288413002

Cr-Commit-Position: refs/heads/master@{#30171}