Vector::insert and Vector::append may use memcpy for POD types.

It will improve performance when POD types are copied, by providing
a missing implementation of VectorCopier and using it in
two places which were previously always taking the "new" allocation
path even though they could have used VectorCopier and used
memcpy for POD types.

NOTRY=true

Review URL: https://codereview.chromium.org/178313002

git-svn-id: svn://svn.chromium.org/blink/trunk@168472 bbb929c8-8fbe-4397-9dbb-9b2b20218538