Remove unnecessary checks in PaintLayer::backgroundIsKnownToBeOpaqueInRect

The PaintLayer::backgroundIsKnownToBeOpaque method is currently
only called by GraphicsLayers on their owning paint layer, or
by the recursive child call. The former is by definition
always called on a self painting layer, while the latter
already has a test to prevent consideration of layers that
do not paint into the backing in question. Hence we can
remove the check for self painting layer status.

These checks cause chicken-and-egg problems when the method
is called by the layer itself in order to identify compositing
reasons. In such cases we are not concerned with self layer
status; we are in fact trying to determine that status.

This is a relanding of issue 2297873002 at patchset 140001
(http://crrev.com/2297873002#ps140001) without the DCHECK that was in the that patch.

R=chrishtr@chromium.org
BUG=381840
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2320713002
Cr-Commit-Position: refs/heads/master@{#417002}

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

@@ -2407,9 +2407,6 @@ bool PaintLayer::paintsWithTransform(GlobalPaintFlags globalPaintFlags) const
 
 bool PaintLayer::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const
 {
-    if (!isSelfPaintingLayer() && !hasSelfPaintingLayerDescendant())
-        return false;
-
     if (paintsWithTransparency(GlobalPaintNormalPhase))
         return false;
 
@@ -2425,14 +2422,15 @@ bool PaintLayer::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect)
     if (paintsWithTransform(GlobalPaintNormalPhase))
         return false;
 
-    // FIXME: Remove this check.
     // This function should not be called when layer-lists are dirty.
-    // It is somehow getting triggered during style update.
+    // TODO(schenney) This check never hits in layout tests or most platforms, but does hit in
+    // PopupBlockerBrowserTest.AllowPopupThroughContentSetting on Win 7 Test Builder.
     if (m_stackingNode->zOrderListsDirty())
         return false;
 
     // FIXME: We currently only check the immediate layoutObject,
-    // which will miss many cases.
+    // which will miss many cases where additional layout objects paint
+    // into this layer.
     if (layoutObject()->backgroundIsKnownToBeOpaqueInRect(localRect))
         return true;
 
@@ -2441,13 +2439,17 @@ bool PaintLayer::backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect)
     if (layoutObject()->hasClipRelatedProperty())
         return false;
 
+    // TODO(schenney): This could be improved by unioning the opaque regions of all the children.
+    // That would require a refactoring because currently children just check they at least
+    // cover the given rect, but a unioning method would require children to compute and report
+    // their rects.
     return childBackgroundIsKnownToBeOpaqueInRect(localRect);
 }
 
 bool PaintLayer::childBackgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const
 {
-    PaintLayerStackingNodeReverseIterator revertseIterator(*m_stackingNode, PositiveZOrderChildren | NormalFlowChildren | NegativeZOrderChildren);
-    while (PaintLayerStackingNode* child = revertseIterator.next()) {
+    PaintLayerStackingNodeReverseIterator reverseIterator(*m_stackingNode, PositiveZOrderChildren | NormalFlowChildren | NegativeZOrderChildren);
+    while (PaintLayerStackingNode* child = reverseIterator.next()) {
         const PaintLayer* childLayer = child->layer();
         // Stop at composited paint boundaries.
         if (childLayer->isPaintInvalidationContainer())