Implement CSS luminance masking using skia SkLumaMaskXfermode class.

Source/core/platform/graphics/GraphicsContext.h

Index: Source/core/platform/graphics/GraphicsContext.h
diff --git a/Source/core/platform/graphics/GraphicsContext.h b/Source/core/platform/graphics/GraphicsContext.h
index 8827fb01029d97c58a2d1eebe82fbd13d024a99f..28b70886f46a694bcf3b6481ee51e7068ecb4cc4 100644
--- a/Source/core/platform/graphics/GraphicsContext.h
+++ b/Source/core/platform/graphics/GraphicsContext.h
@@ -164,6 +164,9 @@ public:
     CompositeOperator compositeOperation() const { return m_state->m_compositeOperator; }
     BlendMode blendModeOperation() const { return m_state->m_blendMode; }
 
+    void setDrawLuminanceMask(bool drawLuminanceMask) { m_state->m_drawLuminanceMask = drawLuminanceMask; }
+    bool drawLuminanceMask() const { return m_state->m_drawLuminanceMask; }
+
     // Change the way document markers are rendered.
     // Any deviceScaleFactor higher than 1.5 is enough to justify setting this flag.
     void setUseHighResMarkers(bool isHighRes) { m_useHighResMarker = isHighRes; }