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

Source/WebCore/platform/graphics/Color.cpp

Index: Source/WebCore/platform/graphics/Color.cpp
diff --git a/Source/WebCore/platform/graphics/Color.cpp b/Source/WebCore/platform/graphics/Color.cpp
index 04dc73ea6ea27a770cea42e6f487bb2eefb45f09..a6b13c7116c5779b279d27a89c5bce5b59d28db4 100644
--- a/Source/WebCore/platform/graphics/Color.cpp
+++ b/Source/WebCore/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);