WTF: Remove RefCountedCustomAllocated<>.

The class template RefCountedCustomAllocated<> is defined in RefCounted.h
but it is not used in Blink at all.

BUG=none
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/59323006

git-svn-id: svn://svn.chromium.org/blink/trunk@161816 bbb929c8-8fbe-4397-9dbb-9b2b20218538