Replace manual pointer arithmetic with WTF::Vector in Gradient::shader()

It's more idiomatic to use a vector for a dynamically-sized array; it also 
provides a convenient opportunity to use inline capacity in the common case
where a gradient has a small number of stops.

BUG=

Review URL: https://codereview.chromium.org/201523002

git-svn-id: svn://svn.chromium.org/blink/trunk@169686 bbb929c8-8fbe-4397-9dbb-9b2b20218538