BitmapImage: Use the cheapest Vector constructor.

Vector() and Vector(0) will result in the same object
but Vector() is cheaper (unless the optimizer manages
to optimize away everything in Vector(size_t) ).

BUG=

Review URL: https://codereview.chromium.org/435913002

git-svn-id: svn://svn.chromium.org/blink/trunk@181084 bbb929c8-8fbe-4397-9dbb-9b2b20218538