Put HashTable::checkKey() in place

This is the partial revert of r155808.

We need to keep 'checkKey()' to catch the possible HashTable misuse
(if empty or deleted keys are attempted to pass) and to make it
clear that such keys are not expected.

The 'checkKey()' calls are not put back to 'lookupForWriting'
and 'fullLookupForWriting' methods since the passed keys are
already checked within the parent calls.

Review URL: https://codereview.chromium.org/216153004

git-svn-id: svn://svn.chromium.org/blink/trunk@170775 bbb929c8-8fbe-4397-9dbb-9b2b20218538