Use Shadow DOM to display fallback content for images

Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp

Index: Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp
diff --git a/Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp b/Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp
index 25d018ab7cce351774e0e189430d9449afebb142..a3ecc480ecf3b497e3699a773c92f3e1fc4688c3 100644
--- a/Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp
+++ b/Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp
@@ -138,7 +138,11 @@ static bool ruleAdditionMightRequireDocumentStyleRecalc(StyleRuleBase* rule)
 
 void StyleSheetInvalidationAnalysis::analyzeStyleSheet(StyleSheetContents* styleSheetContents)
 {
-    ASSERT(!styleSheetContents->isLoading());
+    // Updating the style on the shadow DOM for image fallback content can bring us here when imports
+    // are still getting loaded in the main document. Just need to exit early as we will return here
+    // when the imports finish loading.
+    if (styleSheetContents->isLoading())
+        return;
 
     // See if all rules on the sheet are scoped to some specific ids or classes.
     // Then test if we actually have any of those in the tree at the moment.