Make first-letter style to work with editing

Source/core/rendering/RenderText.cpp

Index: Source/core/rendering/RenderText.cpp
diff --git a/Source/core/rendering/RenderText.cpp b/Source/core/rendering/RenderText.cpp
index 94a73bfa6c33b0177fc3fba9bb091965e1fcddc7..7a9acfdfde1904a31ac6970d7d64407d419b6ea5 100644
--- a/Source/core/rendering/RenderText.cpp
+++ b/Source/core/rendering/RenderText.cpp
@@ -528,7 +528,8 @@ static PositionWithAffinity createPositionWithAffinityForBox(const InlineBox* bo
         affinity = offset > box->caretMinOffset() ? VP_UPSTREAM_IF_POSSIBLE : DOWNSTREAM;
         break;
     }
-    return box->renderer()->createPositionWithAffinity(offset, affinity);
+    int textStartOffset = box->renderer()->isText() ? toRenderText(box->renderer())->textStartOffset() : 0;
+    return box->renderer()->createPositionWithAffinity(offset + textStartOffset, affinity);
 }
 
 static PositionWithAffinity createPositionWithAffinityForBoxAfterAdjustingOffsetForBiDi(const InlineTextBox* box, int offset, ShouldAffinityBeDownstream shouldAffinityBeDownstream)