Introduce WTF_USE_ALLOCATOR macro

Introduce 'WTF_USE_ALLOCATOR' macro and use it in the
container classes having 'Allocator' template argument.

This macro helps to remove repeated code from
the container classes declaration.

Review URL: https://codereview.chromium.org/228043005

git-svn-id: svn://svn.chromium.org/blink/trunk@171134 bbb929c8-8fbe-4397-9dbb-9b2b20218538