ASSERT on renderer lifecycle

Source/core/rendering/RenderBlock.cpp

Index: Source/core/rendering/RenderBlock.cpp
diff --git a/Source/core/rendering/RenderBlock.cpp b/Source/core/rendering/RenderBlock.cpp
index 6236af469fc215673ba017bf059b37b293c38de4..2df31ad2110cb2cd5cc9a09ade6c61be17939f60 100644
--- a/Source/core/rendering/RenderBlock.cpp
+++ b/Source/core/rendering/RenderBlock.cpp
@@ -4036,8 +4036,12 @@ void RenderBlock::createFirstLetterRenderer(RenderObject* firstLetterBlock, Rend
     else
         firstLetter = RenderBlockFlow::createAnonymous(&document());
     firstLetter->setStyle(pseudoStyle);
-    firstLetterContainer->addChild(firstLetter, currentChild);
 
+    // FIXME: The first letter code should not modify the render tree during
+    // layout. crbug.com/370458
+    DeprecatedDisableModifyRenderTreeStructureAsserts disabler;
+
+    firstLetterContainer->addChild(firstLetter, currentChild);
     RenderText* textObj = toRenderText(currentChild);
 
     // The original string is going to be either a generated content string or a DOM node's