Visual change on background-clip changes.

The background-clip property affects background-color visually, so we
need to say a layer is visually different even without an image.

We had a wpt test for this which had a stray line which made the test
always pass.

Bug: 887350
Change-Id: Icc585f9e631485970b6509e015eff1e2007e4f0b
Reviewed-on: https://chromium-review.googlesource.com/1238448
Reviewed-by: Anders Ruud <[email protected]>
Reviewed-by: Fredrik Söderquist <[email protected]>
Commit-Queue: Rune Lillesveen <[email protected]>
Cr-Commit-Position: refs/heads/master@{#593523}

css/css-backgrounds/background-clip-color-repaint.html

@@ -31,5 +31,4 @@
       document.documentElement.classList.remove("reftest-wait");
     });
   });
-  inner.style.backgroundClip = "border-box";
 </script>