Fix misinterpretation of background-position edge in underlying value for animations

third_party/WebKit/LayoutTests/animations/underlying-background-position-edge.html

Index: third_party/WebKit/LayoutTests/animations/underlying-background-position-edge.html
diff --git a/third_party/WebKit/LayoutTests/animations/underlying-background-position-edge.html b/third_party/WebKit/LayoutTests/animations/underlying-background-position-edge.html
new file mode 100644
index 0000000000000000000000000000000000000000..e1268c743a0c6665e552f201d269235363bee46e
--- /dev/null
+++ b/third_party/WebKit/LayoutTests/animations/underlying-background-position-edge.html
@@ -0,0 +1,10 @@
+<script src="../resources/testharness.js"></script>
+<script src="../resources/testharnessreport.js"></script>
+<div id="target"></div>
+<script>
+test(() => {
+  target.style.backgroundPosition = 'right 20px bottom 20px';
+  target.animate({backgroundPosition: 'left 20px top 20px'}, {iterationStart: 0.5, fill: 'forwards'});
+  assert_equals(getComputedStyle(target).backgroundPosition, '50% 50%');
+}, 'Don\'t ignore the specified edge of the underlying background-position value in animations');
+</script>