Add basic tests for multiple position gradient stops

Tests rendering for the the case where <color-stop-length> or
(or <color-stop-angle>) is two <length-percentage> (or
<angle-percentage>.)

Spec:
 https://drafts.csswg.org/css-images-4/#color-stop-syntax

Bug: 707047, 880733
Change-Id: I56bc9b43858b37c003e081ea0ef8588f3da78627
Reviewed-on: https://chromium-review.googlesource.com/1228120
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591717}
4 files changed