Clarify GraphicsContext::beginLayer()/endLayer() have unexpected behaviors.

Source/platform/graphics/GraphicsContext.h

Index: Source/platform/graphics/GraphicsContext.h
diff --git a/Source/platform/graphics/GraphicsContext.h b/Source/platform/graphics/GraphicsContext.h
index e1ea64f874a750e30615d7840d379160a040e096..c0be6569a39d6882b7de8365539a1e4d9203676e 100644
--- a/Source/platform/graphics/GraphicsContext.h
+++ b/Source/platform/graphics/GraphicsContext.h
@@ -109,9 +109,6 @@ public:
     void disableDestructionChecks() { m_disableDestructionChecks = true; }
 #endif
 
-    void saveLayer(const SkRect* bounds, const SkPaint*);
-    void restoreLayer();
-
     bool hasStroke() const { return strokeStyle() != NoStroke && strokeThickness() > 0; }
 
     float strokeThickness() const { return immutableState()->strokeData().thickness(); }
@@ -336,7 +333,10 @@ public:
     };
     void drawLineForDocumentMarker(const FloatPoint&, float width, DocumentMarkerLineStyle);
 
+    // This behaves the same as save(), but in addition it allocates an offscreen buffer.
     void beginTransparencyLayer(float opacity, const FloatRect* = 0);
+    // Apply CompositeOperator when the layer is composited on the backdrop (i.e. endLayer()).
+    // Don't change the current CompositeOperator state.
     void beginLayer(float opacity, CompositeOperator, const FloatRect* = 0, ColorFilter = ColorFilterNone, ImageFilter* = 0);
     void endLayer();
 
@@ -447,6 +447,9 @@ private:
     static void draw2xMarker(SkBitmap*, int);
 #endif
 
+    void saveLayer(const SkRect* bounds, const SkPaint*);
+    void restoreLayer();
+
     // Helpers for drawing a focus ring (drawFocusRing)
     float prepareFocusRingPaint(SkPaint&, const Color&, int width) const;
     void drawFocusRingPath(const SkPath&, const Color&, int width);