Let StyleResolver figure out differences between UA style and resolved style.

Source/core/css/resolver/StyleResolver.cpp

Index: Source/core/css/resolver/StyleResolver.cpp
diff --git a/Source/core/css/resolver/StyleResolver.cpp b/Source/core/css/resolver/StyleResolver.cpp
index fd039ac38b58e3e29613f0e80ec0f852397b575c..8e249cc224d27d0b99962ea3cc107395b0f8b36d 100644
--- a/Source/core/css/resolver/StyleResolver.cpp
+++ b/Source/core/css/resolver/StyleResolver.cpp
@@ -80,7 +80,9 @@
 #include "core/html/HTMLIFrameElement.h"
 #include "core/inspector/InspectorInstrumentation.h"
 #include "core/rendering/RenderView.h"
+#include "core/rendering/style/CachedUAStyle.h"
 #include "core/rendering/style/KeyframeList.h"
+#include "core/rendering/style/UAStyleChange.h"
 #include "core/svg/SVGDocumentExtensions.h"
 #include "core/svg/SVGElement.h"
 #include "platform/RuntimeEnabledFeatures.h"
@@ -526,10 +528,34 @@ PassRefPtr<RenderStyle> StyleResolver::styleForDocument(Document& document)
     return documentStyle.release();
 }
 
+static UAStyleChange uaStyleChange(StyleResolverState& state)
+{
+    const CachedUAStyle* cachedUAStyle = state.cachedUAStyle();
+
+    if (!cachedUAStyle)
+        return UAStyleChange(false);
+
+    UAStyleChange uaStyle(true);
+
+    // Exclude background-repeat from comparison by resetting it.
+    FillLayer backgroundCopy = cachedUAStyle->backgroundLayers;
+    FillLayer backgroundLayersCopy = state.style()->backgroundLayers();
+    backgroundCopy.setRepeatX(NoRepeatFill);
+    backgroundCopy.setRepeatY(NoRepeatFill);
+    backgroundLayersCopy.setRepeatX(NoRepeatFill);
+    backgroundLayersCopy.setRepeatY(NoRepeatFill);
+
+    uaStyle.backgroundChanged = backgroundLayersCopy != backgroundCopy
+        || state.style()->visitedDependentColor(CSSPropertyBackgroundColor) != cachedUAStyle->backgroundColor;
+    uaStyle.borderChanged = state.style()->border() != cachedUAStyle->border;
+
+    return uaStyle;
+}
+
 void StyleResolver::adjustRenderStyle(StyleResolverState& state, Element* element)
 {
     StyleAdjuster adjuster(document().inQuirksMode());
-    adjuster.adjustRenderStyle(state.style(), state.parentStyle(), element, state.cachedUAStyle());
+    adjuster.adjustRenderStyle(state.style(), state.parentStyle(), element, uaStyleChange(state));
 }
 
 // Start loading resources referenced by this style.