Revert of Put HashTable::checkKey() in place (https://codereview.chromium.org/216153004/)

Reason for revert:
Use of uninitalized variables. See error message: http://pastebin.com/zeMPg6Ty
The issue is that when the key of the hash table is a std::pair, we only use the first value for Traits::constructDeletedValue, and the second value will be uninitialized.

HashTranslator::equal compares both value, this patch will always triggers undefined behavior.

Original issue's description:
> 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.
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=170775

TBR=erik.corry@gmail.com,jochen@chromium.org,mikhail.pozdnyakov@intel.com
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/225143002

git-svn-id: svn://svn.chromium.org/blink/trunk@170815 bbb929c8-8fbe-4397-9dbb-9b2b20218538