SVG doesn't recognize ch units.

Source/core/svg/SVGLengthContext.h

Index: Source/core/svg/SVGLengthContext.h
diff --git a/Source/core/svg/SVGLengthContext.h b/Source/core/svg/SVGLengthContext.h
index 81c32d33f686234abd208f101ba632abeeb7c493..93a406481b6c417e09aa3425938a537255036ff4 100644
--- a/Source/core/svg/SVGLengthContext.h
+++ b/Source/core/svg/SVGLengthContext.h
@@ -43,7 +43,8 @@ enum SVGLengthType {
     LengthTypeIN,
     LengthTypePT,
     LengthTypePC,
-    LengthTypeREMS
+    LengthTypeREMS,
+    LengthTypeCHS
 };
 
 enum class SVGLengthMode {
@@ -83,6 +84,9 @@ private:
     float convertValueFromUserUnitsToEXS(float value) const;
     float convertValueFromEXSToUserUnits(float value) const;
 
+    float convertValueFromUserUnitsToCHS(float value) const;
+    float convertValueFromCHSToUserUnits(float value) const;
+
     RawPtrWillBeMember<const SVGElement> m_context;
 };