Remove the Vector::append overload that takes a Vector

Use Vector::appendVector instead.
This is based on webkit CL http://trac.webkit.org/changeset/149579

BUG=NONE

Review URL: https://codereview.chromium.org/150653006

git-svn-id: svn://svn.chromium.org/blink/trunk@167458 bbb929c8-8fbe-4397-9dbb-9b2b20218538