Don't deallocate the HashTable backing explicitly when it is managed by GC.

This leads to double finalization.

NOTRY=true

Review URL: https://codereview.chromium.org/180273022

git-svn-id: svn://svn.chromium.org/blink/trunk@168467 bbb929c8-8fbe-4397-9dbb-9b2b20218538