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

Source/core/platform/graphics/GraphicsContext.cpp

 /*
  * Copyright (C) 2003, 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 255 matching lines @@
     m_state->m_fillPattern.clear();
 }
 
 void GraphicsContext::setShadow(const FloatSize& offset, float blur, const Color& color,
     DrawLooper::ShadowTransformMode shadowTransformMode,
     DrawLooper::ShadowAlphaMode shadowAlphaMode)
 {
     if (paintingDisabled())
         return;
 
-    if (!color.alpha() || (!offset.width() && !offset.height() && !blur)) {
+    if (!color.isValid() || !color.alpha() || (!offset.width() && !offset.height() && !blur)) {
         clearShadow();
         return;
     }
 
     DrawLooper drawLooper;
     drawLooper.addShadow(offset, blur, color, shadowTransformMode, shadowAlphaMode);
     drawLooper.addUnmodifiedContent();
     setDrawLooper(drawLooper);
 }
 
@@ skipping 1570 matching lines @@
 
 void GraphicsContext::didDrawTextInRect(const SkRect& textRect)
 {
     if (m_trackTextRegion) {
         TRACE_EVENT0("skia", "PlatformContextSkia::trackTextRegion");
         m_textRegion.join(textRect);
     }
 }
 
 }