Implement CSS luminance masking using skia SkLumaMaskXfermode class.

Source/core/platform/graphics/GraphicsContextState.h

Index: Source/core/platform/graphics/GraphicsContextState.h
diff --git a/Source/core/platform/graphics/GraphicsContextState.h b/Source/core/platform/graphics/GraphicsContextState.h
index eed9fd502f3f4f55fb75c162e82d5cec5a377ccc..ea27cb770f6ec1929436a3c8f62754e126f68efc 100644
--- a/Source/core/platform/graphics/GraphicsContextState.h
+++ b/Source/core/platform/graphics/GraphicsContextState.h
@@ -65,6 +65,7 @@ private:
         , m_shouldAntialias(true)
         , m_shouldSmoothFonts(true)
         , m_shouldClampToSourceRect(true)
+        , m_drawLuminanceMask(false)
     {
     }
 
@@ -85,6 +86,7 @@ private:
         , m_shouldAntialias(other.m_shouldAntialias)
         , m_shouldSmoothFonts(other.m_shouldSmoothFonts)
         , m_shouldClampToSourceRect(other.m_shouldClampToSourceRect)
+        , m_drawLuminanceMask(other.m_drawLuminanceMask)
     {
     }
 
@@ -138,6 +140,7 @@ private:
     bool m_shouldAntialias : 1;
     bool m_shouldSmoothFonts : 1;
     bool m_shouldClampToSourceRect : 1;
+    bool m_drawLuminanceMask : 1;
 };
 
 } // namespace WebCore