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

Source/core/rendering/RenderBlockLineLayout.cpp

Index: Source/core/rendering/RenderBlockLineLayout.cpp
diff --git a/Source/core/rendering/RenderBlockLineLayout.cpp b/Source/core/rendering/RenderBlockLineLayout.cpp
index 1d0408eed45cf925f959c8dac35128090df9cc11..ce81bc9dde2ab711acd439ac3558ad18748bf8af 100644
--- a/Source/core/rendering/RenderBlockLineLayout.cpp
+++ b/Source/core/rendering/RenderBlockLineLayout.cpp
@@ -1463,7 +1463,7 @@ RootInlineBox* RenderBlock::createLineBoxesFromBidiRuns(unsigned bidiLevel, Bidi
     // text selection in RTL boxes would not work as expected.
     if (isSVGRootInlineBox) {
         ASSERT(isSVGText());
-        static_cast<SVGRootInlineBox*>(lineBox)->computePerCharacterLayoutInformation();
+        toSVGRootInlineBox(lineBox)->computePerCharacterLayoutInformation();
     }
 
     // Compute our overflow now.