Now that it is no longer needed, this also removes the invalid inclusion
of "object-inl.h" within the "unique.h" header file.

Note that this change still leaves 2 violations of that rule in the
code, checked with the "tools/check-inline-includes.sh" tool.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1321223002

Cr-Commit-Position: refs/heads/master@{#30503}