Improved the reporting of background color information.

third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp

Index: third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
diff --git a/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp b/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
index 78acfb0d75c97ab3fd9c16728662883efc8baebf..0dc0fea85520b55967942ac60bbacbdef6748bf6 100644
--- a/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
+++ b/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
@@ -839,12 +839,30 @@ RGBA32 AXLayoutObject::backgroundColor() const
     if (!getLayoutObject())
         return AXNodeObject::backgroundColor();
 
-    const ComputedStyle* style = getLayoutObject()->style();
-    if (!style || !style->hasBackground())
-        return AXNodeObject::backgroundColor();
+    Color blendedColor = Color::transparent;
+    for (LayoutObject* layoutObject = getLayoutObject(); layoutObject;

[aboxhall, 2016/05/24 17:06:34]

Unfortunately, walking up the parent chain often isn't sufficient because of
absolute positioning.

I wrote code for this in devtools:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit...

I wonder if we can reuse that here, somehow?

+        layoutObject = layoutObject->parent()) {
+        const ComputedStyle* style = layoutObject->style();
+        if (!style || !style->hasBackground())
+            continue;
 
-    Color color = style->visitedDependentColor(CSSPropertyBackgroundColor);
-    return color.rgb();
+        Color currentColor = style->visitedDependentColor(CSSPropertyBackgroundColor);
+        blendedColor = blendedColor.blend(currentColor);
+        // Continue blending until we get opacity to more than 50%
+        if (blendedColor.alpha() > 128)

[aboxhall, 2016/05/24 17:06:34]

I share Dominic's concern here: for example, 51% red over blue appears purple,
not blue.

+            break;
+    }
+
+    // If we still have a low level of opacity, blend in the document base color.
+    if (blendedColor.alpha() <= 128) {
+        FrameView* view = documentFrameView();
+        if (view) {
+            Color documentBaseColor = view->baseBackgroundColor();
+            blendedColor = blendedColor.blend(documentBaseColor);
+        }
+    }
+
+    return blendedColor.rgb();
 }
 
 RGBA32 AXLayoutObject::color() const
@@ -1678,11 +1696,11 @@ Document* AXLayoutObject::getDocument() const
 
 FrameView* AXLayoutObject::documentFrameView() const
 {
-    if (!m_layoutObject)
-        return 0;
+    if (!getLayoutObject())
+        return nullptr;
 
     // this is the LayoutObject's Document's LocalFrame's FrameView
-    return m_layoutObject->document().view();
+    return getLayoutObject()->document().view();
 }
 
 Element* AXLayoutObject::anchorElement() const