Fix incorrect animation for some values of background-position.

LayoutTests/animations/interpolation/background-position-origin-interpolation-expected.txt

Index: LayoutTests/animations/interpolation/background-position-origin-interpolation-expected.txt
diff --git a/LayoutTests/animations/interpolation/background-position-origin-interpolation-expected.txt b/LayoutTests/animations/interpolation/background-position-origin-interpolation-expected.txt
new file mode 100644
index 0000000000000000000000000000000000000000..92dcad16445a072465529bc1e8c96b55295279bb
--- /dev/null
+++ b/LayoutTests/animations/interpolation/background-position-origin-interpolation-expected.txt
@@ -0,0 +1,46 @@
+PASS: background-position from [center center] to [left 20px top 20px] was [50% 50%] at 0
+PASS: background-position from [center center] to [left 20px top 20px] was [calc(37.5% + 5px) calc(37.5% + 5px)] at 0.25
+PASS: background-position from [center center] to [left 20px top 20px] was [calc(25% + 10px) calc(25% + 10px)] at 0.5
+PASS: background-position from [center center] to [left 20px top 20px] was [calc(12.5% + 15px) calc(12.5% + 15px)] at 0.75
+PASS: background-position from [center center] to [left 20px top 20px] was [20px 20px] at 1
+PASS: background-position from [center center] to [center top 20px] was [50% 50%] at 0
+PASS: background-position from [center center] to [center top 20px] was [50% calc(37.5% + 5px)] at 0.25
+PASS: background-position from [center center] to [center top 20px] was [50% calc(25% + 10px)] at 0.5
+PASS: background-position from [center center] to [center top 20px] was [50% calc(12.5% + 15px)] at 0.75
+PASS: background-position from [center center] to [center top 20px] was [50% 20px] at 1
+PASS: background-position from [center center] to [right 20px top 20px] was [50% 50%] at 0
+PASS: background-position from [center center] to [right 20px top 20px] was [calc(37.5% + ((100% - 20px) * 0.25)) calc(37.5% + 5px)] at 0.25
+PASS: background-position from [center center] to [right 20px top 20px] was [calc(25% + ((100% - 20px) * 0.5)) calc(25% + 10px)] at 0.5
+PASS: background-position from [center center] to [right 20px top 20px] was [calc(12.5% + ((100% - 20px) * 0.75)) calc(12.5% + 15px)] at 0.75
+PASS: background-position from [center center] to [right 20px top 20px] was [calc(100% - 20px) 20px] at 1
+PASS: background-position from [center center] to [left 20px center] was [50% 50%] at 0
+PASS: background-position from [center center] to [left 20px center] was [calc(37.5% + 5px) 50%] at 0.25
+PASS: background-position from [center center] to [left 20px center] was [calc(25% + 10px) 50%] at 0.5
+PASS: background-position from [center center] to [left 20px center] was [calc(12.5% + 15px) 50%] at 0.75
+PASS: background-position from [center center] to [left 20px center] was [20px 50%] at 1
+PASS: background-position from [center center] to [center center] was [50% 50%] at 0
+PASS: background-position from [center center] to [center center] was [50% 50%] at 0.25
+PASS: background-position from [center center] to [center center] was [50% 50%] at 0.5
+PASS: background-position from [center center] to [center center] was [50% 50%] at 0.75
+PASS: background-position from [center center] to [center center] was [50% 50%] at 1
+PASS: background-position from [center center] to [right 20px center] was [50% 50%] at 0
+PASS: background-position from [center center] to [right 20px center] was [calc(37.5% + ((100% - 20px) * 0.25)) 50%] at 0.25
+PASS: background-position from [center center] to [right 20px center] was [calc(25% + ((100% - 20px) * 0.5)) 50%] at 0.5
+PASS: background-position from [center center] to [right 20px center] was [calc(12.5% + ((100% - 20px) * 0.75)) 50%] at 0.75
+PASS: background-position from [center center] to [right 20px center] was [calc(100% - 20px) 50%] at 1
+PASS: background-position from [center center] to [left 20px bottom 20px] was [50% 50%] at 0
+PASS: background-position from [center center] to [left 20px bottom 20px] was [calc(37.5% + 5px) calc(37.5% + ((100% - 20px) * 0.25))] at 0.25
+PASS: background-position from [center center] to [left 20px bottom 20px] was [calc(25% + 10px) calc(25% + ((100% - 20px) * 0.5))] at 0.5
+PASS: background-position from [center center] to [left 20px bottom 20px] was [calc(12.5% + 15px) calc(12.5% + ((100% - 20px) * 0.75))] at 0.75
+PASS: background-position from [center center] to [left 20px bottom 20px] was [20px calc(100% - 20px)] at 1
+PASS: background-position from [center center] to [center bottom 20px] was [50% 50%] at 0
+PASS: background-position from [center center] to [center bottom 20px] was [50% calc(37.5% + ((100% - 20px) * 0.25))] at 0.25
+PASS: background-position from [center center] to [center bottom 20px] was [50% calc(25% + ((100% - 20px) * 0.5))] at 0.5
+PASS: background-position from [center center] to [center bottom 20px] was [50% calc(12.5% + ((100% - 20px) * 0.75))] at 0.75
+PASS: background-position from [center center] to [center bottom 20px] was [50% calc(100% - 20px)] at 1
+PASS: background-position from [center center] to [right 20px bottom 20px] was [50% 50%] at 0
+PASS: background-position from [center center] to [right 20px bottom 20px] was [calc(37.5% + ((100% - 20px) * 0.25)) calc(37.5% + ((100% - 20px) * 0.25))] at 0.25
+PASS: background-position from [center center] to [right 20px bottom 20px] was [calc(25% + ((100% - 20px) * 0.5)) calc(25% + ((100% - 20px) * 0.5))] at 0.5
+PASS: background-position from [center center] to [right 20px bottom 20px] was [calc(12.5% + ((100% - 20px) * 0.75)) calc(12.5% + ((100% - 20px) * 0.75))] at 0.75
+PASS: background-position from [center center] to [right 20px bottom 20px] was [calc(100% - 20px) calc(100% - 20px)] at 1
+