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/1238448Reviewed-by: Anders Ruud <andruud@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593523}

third_party/WebKit/LayoutTests/external/wpt/css/css-backgrounds/background-clip-color-repaint.html

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

third_party/blink/renderer/core/style/fill_layer.cc

@@ -189,6 +189,8 @@ bool FillLayer::VisuallyEqual(const FillLayer& o) const {
   if (image_ || o.image_) {
     if (!LayerPropertiesEqual(o))
       return false;
+  } else if (clip_ != o.clip_) {
+    return false;
   }
   if (next_ && o.next_)
     return next_->VisuallyEqual(*o.next_);