Change the caret color according to the lightness of the background color.

Source/core/platform/graphics/Color.h

Index: Source/core/platform/graphics/Color.h
diff --git a/Source/core/platform/graphics/Color.h b/Source/core/platform/graphics/Color.h
index 87530607801f840b0c3919ff7bf76a37ccaee959..f0ed2a41a355650319260a4ced679b5764d8aa0c 100644
--- a/Source/core/platform/graphics/Color.h
+++ b/Source/core/platform/graphics/Color.h
@@ -103,6 +103,8 @@ public:
     void getRGBA(double& r, double& g, double& b, double& a) const;
     void getHSL(double& h, double& s, double& l) const;
 
+    double lightness() const;
+
     Color light() const;
     Color dark() const;