Use Shadow DOM to display fallback content for images

Source/core/css/invalidation/StyleSheetInvalidationAnalysis.cpp

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 120 matching lines @@
     case StyleRule::Keyframe:
     case StyleRule::Style:
         break;
     }
     ASSERT_NOT_REACHED();
     return true;
 }
 
 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.
     const WillBeHeapVector<RefPtrWillBeMember<StyleRuleImport> >& importRules = styleSheetContents->importRules();
     for (unsigned i = 0; i < importRules.size(); ++i) {
         if (!importRules[i]->styleSheet())
             continue;
         analyzeStyleSheet(importRules[i]->styleSheet());
         if (m_dirtiesAllStyle)
             return;
@@ skipping 55 matching lines @@
             element->setNeedsStyleRecalc(SubtreeStyleChange, StyleChangeReasonForTracing::create(StyleChangeReason::StyleSheetChange));
             // The whole subtree is now invalidated, we can skip to the next sibling.
             element = ElementTraversal::nextSkippingChildren(*element);
             continue;
         }
         element = ElementTraversal::next(*element);
     }
 }
 
 }