Exit early in HashTable destructor if it is empty.

It is common for a HashTable to never get used. By adding an early exit, we
avoid an expensive function call to deallocateTable(). Even if that were to get
inlined, the expensive call to fastFree(NULL) cannot be avoided.

Review URL: https://chromiumcodereview.appspot.com/23929004

git-svn-id: svn://svn.chromium.org/blink/trunk@158130 bbb929c8-8fbe-4397-9dbb-9b2b20218538