[CSS Grid Layout] Use CSS_FR for flexible lengths instead of CSS_DIMENSION

Source/core/css/CSSPrimitiveValue.cpp

Index: Source/core/css/CSSPrimitiveValue.cpp
diff --git a/Source/core/css/CSSPrimitiveValue.cpp b/Source/core/css/CSSPrimitiveValue.cpp
index a77565b0bd66985417327c76864ae20775661868..9a5741efa55ed33871e858c231ce883e78eccf63 100644
--- a/Source/core/css/CSSPrimitiveValue.cpp
+++ b/Source/core/css/CSSPrimitiveValue.cpp
@@ -988,7 +988,8 @@ String CSSPrimitiveValue::customCssText(CssTextFormattingFlags formattingFlag) c
             text = formatNumber(m_value.num, "turn");
             break;
         case CSS_DIMENSION:
-            text = m_value.string;
+            // FIXME: We currently don't handle CSS_DIMENSION properly as we don't store
+            // the actual dimension, just the numeric value as a string.
             break;
         case CSS_STRING:
             text = formattingFlag == AlwaysQuoteCSSString ? quoteCSSString(m_value.string) : quoteCSSStringIfNeeded(m_value.string);