Introduce toSVGInlineFlowBox() and use it

Source/core/rendering/svg/SVGRootInlineBox.cpp

Index: Source/core/rendering/svg/SVGRootInlineBox.cpp
diff --git a/Source/core/rendering/svg/SVGRootInlineBox.cpp b/Source/core/rendering/svg/SVGRootInlineBox.cpp
index 03b364bc93ef1fd9e2a65ed69fdccd00a2ffa6b6..f81e128044e92e00749640f166ad0789997e0571 100644
--- a/Source/core/rendering/svg/SVGRootInlineBox.cpp
+++ b/Source/core/rendering/svg/SVGRootInlineBox.cpp
@@ -50,7 +50,7 @@ void SVGRootInlineBox::paint(PaintInfo& paintInfo, const LayoutPoint&, LayoutUni
             if (child->isSVGInlineTextBox())
                 toSVGInlineTextBox(child)->paintSelectionBackground(childPaintInfo);
             else if (child->isSVGInlineFlowBox())
-                static_cast<SVGInlineFlowBox*>(child)->paintSelectionBackground(childPaintInfo);
+                toSVGInlineFlowBox(child)->paintSelectionBackground(childPaintInfo);
         }
     }
 
@@ -112,9 +112,7 @@ void SVGRootInlineBox::layoutCharactersInTextBoxes(InlineFlowBox* start, SVGText
             if (!node)
                 continue;
 
-            ASSERT_WITH_SECURITY_IMPLICATION(child->isInlineFlowBox());
-
-            SVGInlineFlowBox* flowBox = static_cast<SVGInlineFlowBox*>(child);
+            SVGInlineFlowBox* flowBox = toSVGInlineFlowBox(child);
             bool isTextPath = node->hasTagName(SVGNames::textPathTag);
             if (isTextPath) {
                 // Build text chunks for all <textPath> children, using the line layout algorithm.
@@ -152,9 +150,7 @@ void SVGRootInlineBox::layoutChildBoxes(InlineFlowBox* start, FloatRect* childRe
             if (!child->renderer()->node())
                 continue;
 
-            ASSERT_WITH_SECURITY_IMPLICATION(child->isInlineFlowBox());
-
-            SVGInlineFlowBox* flowBox = static_cast<SVGInlineFlowBox*>(child);
+            SVGInlineFlowBox* flowBox = toSVGInlineFlowBox(child);
             layoutChildBoxes(flowBox);
 
             boxRect = flowBox->calculateBoundaries();