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

Source/core/platform/graphics/Color.cpp

Index: Source/core/platform/graphics/Color.cpp
diff --git a/Source/core/platform/graphics/Color.cpp b/Source/core/platform/graphics/Color.cpp
index c6bdeb25cdf5afa5f9bd920d10fa0de997b963a3..53b4c115134e714c642643f8bd0d1b7514272beb 100644
--- a/Source/core/platform/graphics/Color.cpp
+++ b/Source/core/platform/graphics/Color.cpp
@@ -414,6 +414,14 @@ void Color::getHSL(double& hue, double& saturation, double& lightness) const
         saturation = ((max - min) / (2.0 - (max + min)));
 }
 
+double Color::lightness() const
+{
+    double hue, saturation, lightness;
+    getHSL(hue, saturation, lightness);
+
+    return lightness;
+}
+
 Color colorFromPremultipliedARGB(RGBA32 pixelColor)
 {
     int alpha = alphaChannel(pixelColor);