Allow for optimized code map to have zero entries.

Allow for optimized code map to have zero entries.

This allows the optimized code map to contain no context-dependent
entries, but still hold one context-independent entry. This is a
precursor to extending the lifetime of the context-independent entry.

R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/1249543005

Cr-Commit-Position: refs/heads/master@{#29788}