Bug 1774261 part 2: Simplify nsCSSRendering::FindBackgroundFrame to directly return its result instead of using an outparam. r=emilio

This patch doesn't change behavior.

This is similar to the previous patch in this series, but now for
FindBackgroundFrame().

Differential Revision: https://phabricator.services.mozilla.com/D149338

layout/generic/nsCanvasFrame.cpp

@@ -461,13 +461,15 @@ void nsCanvasFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
     ComputedStyle* bg = nullptr;
     nsIFrame* dependentFrame = nullptr;
     bool isThemed = IsThemed();
-    if (!isThemed &&
-        nsCSSRendering::FindBackgroundFrame(this, &dependentFrame)) {
+    if (!isThemed) {
+      dependentFrame = nsCSSRendering::FindBackgroundFrame(this);
+      if (dependentFrame) {
         bg = dependentFrame->Style();
         if (dependentFrame == this) {
           dependentFrame = nullptr;
         }
       }
+    }
 
     if (isThemed) {
       aLists.BorderBackground()

layout/painting/nsCSSRendering.cpp

@@ -1138,8 +1138,8 @@ auto nsCSSRendering::FindNonTransparentBackgroundFrame(nsIFrame* aFrame,
     }
 
     if (IsCanvasFrame(frame)) {
-      nsIFrame* bgFrame = nullptr;
-      if (FindBackgroundFrame(frame, &bgFrame) &&
+      nsIFrame* bgFrame = FindBackgroundFrame(frame);
+      if (bgFrame &&
           NS_GET_A(bgFrame->StyleBackground()->BackgroundColor(bgFrame))) {
         return {bgFrame, false, true};
       }
@@ -1273,22 +1273,24 @@ inline bool FindElementBackground(const nsIFrame* aForFrame,
   return !htmlBG->IsTransparent(aRootElementFrame);
 }
 
-bool nsCSSRendering::FindBackgroundFrame(const nsIFrame* aForFrame,
-                                         nsIFrame** aBackgroundFrame) {
+nsIFrame* nsCSSRendering::FindBackgroundFrame(const nsIFrame* aForFrame) {
   nsIFrame* rootElementFrame =
       aForFrame->PresShell()->FrameConstructor()->GetRootElementStyleFrame();
   if (IsCanvasFrame(aForFrame)) {
-    *aBackgroundFrame = FindCanvasBackgroundFrame(aForFrame, rootElementFrame);
-    return true;
+    return FindCanvasBackgroundFrame(aForFrame, rootElementFrame);
   }
 
-  *aBackgroundFrame = const_cast<nsIFrame*>(aForFrame);
-  return FindElementBackground(aForFrame, rootElementFrame);
+  // XXXdholbert FindElementBackground will be renamed for clarity later
+  // in this series.
+  if (FindElementBackground(aForFrame, rootElementFrame)) {
+    return const_cast<nsIFrame*>(aForFrame);
+  }
+
+  return nullptr;
 }
 
 ComputedStyle* nsCSSRendering::FindBackground(const nsIFrame* aForFrame) {
-  nsIFrame* backgroundFrame = nullptr;
-  if (FindBackgroundFrame(aForFrame, &backgroundFrame)) {
+  if (auto* backgroundFrame = FindBackgroundFrame(aForFrame)) {
     return backgroundFrame->Style();
   }
   return nullptr;

layout/painting/nsCSSRendering.h

@@ -292,8 +292,7 @@ struct nsCSSRendering {
    *         background.
    */
   static mozilla::ComputedStyle* FindBackground(const nsIFrame* aForFrame);
-  static bool FindBackgroundFrame(const nsIFrame* aForFrame,
-                                  nsIFrame** aBackgroundFrame);
+  static nsIFrame* FindBackgroundFrame(const nsIFrame* aForFrame);
 
   /**
    * As FindBackground, but the passed-in frame is known to be a root frame

layout/painting/nsDisplayList.cpp

@@ -2957,8 +2957,8 @@ nsDisplayBackgroundImage::~nsDisplayBackgroundImage() {
 }
 
 static nsIFrame* GetBackgroundComputedStyleFrame(nsIFrame* aFrame) {
-  nsIFrame* f;
-  if (!nsCSSRendering::FindBackgroundFrame(aFrame, &f)) {
+  nsIFrame* f = nsCSSRendering::FindBackgroundFrame(aFrame);
+  if (!f) {
     // We don't want to bail out if moz-appearance is set on a root
     // node. If it has a parent content node, bail because it's not
     // a root, other wise keep going in order to let the theme stuff