This removes the requirement for handles as arguments, but also removes
concurrency support, which is not being used at the moment.

Supporting concurrency could be done by introducing a sibling class to
IdentityMap that includes RelocationLock on method calls.

R=bmeurer@chromium.org, ulan@chromium.org

Review URL: https://codereview.chromium.org/1419563004

Cr-Commit-Position: refs/heads/master@{#31510}