Removes the DONT_DELETE enum bit from the properties to make them
configurable.

Also, updates the regress-typedarray-length test to --
- Check for true boolean return value on deletion of these
  properties.
- Check for undefined return value on trying to access these
  properties after deletion.

BUG=v8:4902
LOG=Y

Review-Url: https://codereview.chromium.org/2001393004
Cr-Commit-Position: refs/heads/master@{#36528}