Add mutableComponent() interface to InterpolationValue

Source/core/animation/PrimitiveInterpolation.h

Index: Source/core/animation/PrimitiveInterpolation.h
diff --git a/Source/core/animation/PrimitiveInterpolation.h b/Source/core/animation/PrimitiveInterpolation.h
index ada198f1620537400baca6e805b7bdde6bc0bb27..a9cad544ca70d206ea540949b59336a21762ae6a 100644
--- a/Source/core/animation/PrimitiveInterpolation.h
+++ b/Source/core/animation/PrimitiveInterpolation.h
@@ -59,7 +59,7 @@ private:
         ASSERT(result);
         ASSERT(&result->type() == &m_type);
         ASSERT(result->nonInterpolableValue() == m_nonInterpolableValue.get());
-        m_start->interpolate(*m_end, fraction, result->interpolableValue());
+        m_start->interpolate(*m_end, fraction, *result->mutableComponent().interpolableValue);
     }
 
     double interpolateUnderlyingFraction(double start, double end, double fraction) const final { return blend(start, end, fraction); }