Remove HashTable debug checks.

I'm not aware that they've ever been particularly useful or prolific at
catching issues, and conversely add to code complexity and debug runtime.

Removing them gets rid of 20MB (about 1%) of Debug chrome binary size.

In the process, a redundant (duplicated) check against iterator.end() was
noted and removed from both HashSet and HashMap, shaving 10KB off the
Release chrome binary, and presumably also eliminating a branch from a
reasonably hot path.

Review URL: https://chromiumcodereview.appspot.com/22646004

git-svn-id: svn://svn.chromium.org/blink/trunk@155808 bbb929c8-8fbe-4397-9dbb-9b2b20218538