Change background-repeat default to no-repeat

Previously, if you had a generated background image and a translucent (or
rounded) border, we would draw the image nine times because we would inflate
the fill rect to draw behind the border. With no-repeat, we only draw the image
once.

This saves 4.5% of record time on flights-app.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/844193009

examples/flights-app/app-toast.sky

@@ -9,7 +9,6 @@
       right: 32px;
       max-width: 55%;
       box-shadow: 0px 0px 12px 2px rgba(22, 22, 22, 0.4);
-      background-color: #b0281a;
       background-image: linear-gradient(#E5D658, #DFCF43);
       border: 1px solid #AEA477;
       padding: 6px;