WebVector::initialize{From} should bounds check its size parameter

BUG=366781
R=jschuh@chromium.org

Review URL: https://codereview.chromium.org/256623004

git-svn-id: svn://svn.chromium.org/blink/trunk@172843 bbb929c8-8fbe-4397-9dbb-9b2b20218538