Use macros instead of specializing VectorTraits explicitly

The existing 'WTF_ALLOW_MOVE_INIT_AND_COMPARE_WITH_MEM_FUNCTIONS'
or the newly added 'WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS'
macros should be used instead of specializing VectorTraits
explicitly, in order to improve code readability and decrease
the number of LOC used.

Review URL: https://codereview.chromium.org/249363002

git-svn-id: svn://svn.chromium.org/blink/trunk@172381 bbb929c8-8fbe-4397-9dbb-9b2b20218538