Cherry-pick b33aded. rdar://problem/93689838

    ASSERTION FAILED: firstChild():[ macOS ] media/track/track-cues-missed.html is a flaky test.
    https://bugs.webkit.org/show_bug.cgi?id=258183
    rdar://110876540

    Reviewed by Andy Estes.

    * Source/WebCore/rendering/RenderVTTCue.cpp:
    (WebCore::RenderVTTCue::isOutside const): Return early if firstChild() return null.
    (WebCore::RenderVTTCue::isOverlapping const): Ditto.
    (WebCore::RenderVTTCue::overlappingObject const): Ditto.
    (WebCore::RenderVTTCue::moveIfNecessaryToKeepWithinContainer): Ditto.
    (WebCore::RenderVTTCue::findNonOverlappingPosition const): Ditto.

    Canonical link: https://commits.webkit.org/265420@main
Identifier: 265377.2@safari-7616.1.20-branch

Source/WebCore/rendering/RenderVTTCue.cpp

@@ -76,7 +76,6 @@ void RenderVTTCue::layout()
 
 bool RenderVTTCue::initializeLayoutParameters(LayoutUnit& step, LayoutUnit& position)
 {
-    ASSERT(firstChild());
     if (!firstChild())
         return false;
 
@@ -158,6 +157,9 @@ void RenderVTTCue::placeBoxInDefaultPosition(LayoutUnit position, bool& switched
 
 bool RenderVTTCue::isOutside() const
 {
+    if (!firstChild())
+        return false;
+
     return !rectIsWithinContainer(backdropBox().absoluteBoundingBoxRect());
 }
 
@@ -169,11 +171,16 @@ bool RenderVTTCue::rectIsWithinContainer(const IntRect& rect) const
 
 bool RenderVTTCue::isOverlapping() const
 {
+    if (!firstChild())
+        return false;
+
     return overlappingObject();
 }
 
 RenderVTTCue* RenderVTTCue::overlappingObject() const
 {
+    ASSERT(firstChild());
+
     return overlappingObjectForRect(backdropBox().absoluteBoundingBoxRect());
 }
 
@@ -257,6 +264,9 @@ bool RenderVTTCue::switchDirection(bool& switched, LayoutUnit& step)
 
 void RenderVTTCue::moveIfNecessaryToKeepWithinContainer()
 {
+    if (!firstChild())
+        return;
+
     IntRect containerRect = containingBlock()->absoluteBoundingBoxRect();
     IntRect cueRect = backdropBox().absoluteBoundingBoxRect();
 
@@ -287,6 +297,9 @@ void RenderVTTCue::moveIfNecessaryToKeepWithinContainer()
 
 bool RenderVTTCue::findNonOverlappingPosition(int& newX, int& newY) const
 {
+    if (!firstChild())
+        return false;
+
     newX = x();
     newY = y();
     IntRect srcRect = backdropBox().absoluteBoundingBoxRect();
@@ -407,6 +420,7 @@ RenderBlockFlow& RenderVTTCue::backdropBox() const
 
 RenderInline& RenderVTTCue::cueBox() const
 {
+    ASSERT(firstChild());
     return downcast<RenderInline>(*backdropBox().firstChild());
 }