Introduce a macro enabling 'SimpleClassVectorTraits'

Introduce a 'WTF_ALLOW_MOVE_INIT_AND_COMPARE_WITH_MEM_FUNCTIONS'
macro enabling 'SimpleClassVectorTraits' to improve code
readability.

Review URL: https://codereview.chromium.org/240213009

git-svn-id: svn://svn.chromium.org/blink/trunk@172136 bbb929c8-8fbe-4397-9dbb-9b2b20218538