Minor Gradient cleanup.

There is no need to manipulate the sorted state externally, we can
efficiently detect sorted color stops on the fly. This may also avoid
a sort pass in some cases.

R=pdr@chromium.org,schenney@chromium.org,rob.buis@samsung.com

Review URL: https://codereview.chromium.org/206493002

git-svn-id: svn://svn.chromium.org/blink/trunk@169660 bbb929c8-8fbe-4397-9dbb-9b2b20218538