Promote opaque fixed position elements.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index 0f1dcb765cacd224c61f36b898d72c3dbcca6389..adc426661d34494263d97a0b7c86df47ea099c77 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -2448,7 +2448,12 @@ bool PaintLayer::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect)
         return false;
 
     // FIXME: Handle simple transforms.
-    if (paintsWithTransform(GlobalPaintNormalPhase))
+    if (transform() && compositingState() != PaintsIntoOwnBacking)
+        return false;
+
+    if (!RuntimeEnabledFeatures::compositeOpaqueFixedPositionEnabled()
+        && layoutObject()->style()->position() == FixedPosition
+        && compositingState() != PaintsIntoOwnBacking)
         return false;
 
     // This function should not be called when layer-lists are dirty.