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

Source/core/inspector/InspectorFrontendHost.cpp

Index: Source/core/inspector/InspectorFrontendHost.cpp
diff --git a/Source/core/inspector/InspectorFrontendHost.cpp b/Source/core/inspector/InspectorFrontendHost.cpp
index 48347801c2e15ab6b465659a74393b69fa0e255b..9df3e41bfbfe8aa7bb3ced0894763d60a53cb095 100644
--- a/Source/core/inspector/InspectorFrontendHost.cpp
+++ b/Source/core/inspector/InspectorFrontendHost.cpp
@@ -282,13 +282,13 @@ String InspectorFrontendHost::loadResourceSynchronously(const String& url)
 String InspectorFrontendHost::getSelectionBackgroundColor()
 {
     Color color = RenderTheme::theme().activeSelectionBackgroundColor();
-    return color != Color::transparent ? color.serialized() : "";
+    return color.isValid() ? color.serialized() : "";
 }
 
 String InspectorFrontendHost::getSelectionForegroundColor()
 {
     Color color = RenderTheme::theme().activeSelectionForegroundColor();
-    return color != Color::transparent ? color.serialized() : "";
+    return color.isValid() ? color.serialized() : "";
 }
 
 bool InspectorFrontendHost::supportsFileSystems()