Make style building for 'font-size' less custom.

Source/core/editing/EditingStyle.cpp

Index: Source/core/editing/EditingStyle.cpp
diff --git a/Source/core/editing/EditingStyle.cpp b/Source/core/editing/EditingStyle.cpp
index 6f064824d50c52604c130d6fd308d863dfe1a343..0a4ea46b031c9d58ae022f1d21f4feaf9f775cb2 100644
--- a/Source/core/editing/EditingStyle.cpp
+++ b/Source/core/editing/EditingStyle.cpp
@@ -1637,8 +1637,7 @@ int legacyFontSizeFromCSSValue(Document* document, CSSPrimitiveValue* value, Fix
         int pixelFontSize = value->getIntValue(CSSPrimitiveValue::CSS_PX);
         int legacyFontSize = FontSize::legacyFontSize(document, pixelFontSize, fixedPitchFontType);
         // Use legacy font size only if pixel value matches exactly to that of legacy font size.
-        CSSValueID cssPrimitiveEquivalent = static_cast<CSSValueID>(legacyFontSize - 1 + CSSValueXSmall);
-        if (mode == AlwaysUseLegacyFontSize || FontSize::fontSizeForKeyword(document, cssPrimitiveEquivalent, fixedPitchFontType) == pixelFontSize)
+        if (mode == AlwaysUseLegacyFontSize || FontSize::fontSizeForKeyword(document, legacyFontSize, fixedPitchFontType) == pixelFontSize)
             return legacyFontSize;
 
         return 0;