Add toSVGRootInlineBox(), and use it as much as we can

Source/core/rendering/svg/RenderSVGText.cpp

Index: Source/core/rendering/svg/RenderSVGText.cpp
diff --git a/Source/core/rendering/svg/RenderSVGText.cpp b/Source/core/rendering/svg/RenderSVGText.cpp
index 6552c0bc917c795adbdac0aa8c33bd6cd5baf1b1..24eb0a037d02278e1c8eabf59ecdf4a23743195a 100644
--- a/Source/core/rendering/svg/RenderSVGText.cpp
+++ b/Source/core/rendering/svg/RenderSVGText.cpp
@@ -468,11 +468,10 @@ PositionWithAffinity RenderSVGText::positionForPoint(const LayoutPoint& pointInC
     if (!rootBox)
         return createPositionWithAffinity(0, DOWNSTREAM);
 
-    ASSERT_WITH_SECURITY_IMPLICATION(rootBox->isSVGRootInlineBox());
     ASSERT(!rootBox->nextRootBox());
     ASSERT(childrenInline());
 
-    InlineBox* closestBox = static_cast<SVGRootInlineBox*>(rootBox)->closestLeafChildForPosition(pointInContents);
+    InlineBox* closestBox = toSVGRootInlineBox(rootBox)->closestLeafChildForPosition(pointInContents);
     if (!closestBox)
         return createPositionWithAffinity(0, DOWNSTREAM);