Revert r154797: "Move isValid/isCurrentColor from Color to StyleColor"

Source/core/platform/graphics/ShadowBlur.cpp

Index: Source/core/platform/graphics/ShadowBlur.cpp
diff --git a/Source/core/platform/graphics/ShadowBlur.cpp b/Source/core/platform/graphics/ShadowBlur.cpp
index 4028b6dbc3e036c8139813dd9582aa4e8865c91b..572c97ca97bda6ce7578cdb74c4c1591715095ff 100644
--- a/Source/core/platform/graphics/ShadowBlur.cpp
+++ b/Source/core/platform/graphics/ShadowBlur.cpp
@@ -55,7 +55,7 @@ void ShadowBlur::updateShadowBlurValues()
     m_blurRadius = m_blurRadius.shrunkTo(FloatSize(128, 128));
 
     // The type of shadow is decided by the blur radius, shadow offset, and shadow color.
-    if (!m_color.alpha()) {
+    if (!m_color.isValid() || !m_color.alpha()) {
         // Can't paint the shadow with invalid or invisible color.
         m_type = NoShadow;
     } else if (m_blurRadius.width() > 0 || m_blurRadius.height() > 0) {