use Descriptor struct to encapsulate all the common paramaeters between our various gradient types. If we like it, might promote it to the public API.

BUG=
R=bsalomon@google.com

Review URL: https://codereview.chromium.org/15733007

git-svn-id: http://skia.googlecode.com/svn/trunk@9260 2bbb7eff-a529-9590-31e7-b0007b416f81
12 files changed