Enable the initial Vector size to be set with a define.

This will enable quick perf experiments of different values without having to
wait for the experiment to hit (and depart) Chromium via a Blink roll. It will
be possible to simply set a gyp setting in Chromium.

Review URL: https://chromiumcodereview.appspot.com/23804003

git-svn-id: svn://svn.chromium.org/blink/trunk@157284 bbb929c8-8fbe-4397-9dbb-9b2b20218538