Add comments to SkRecordDraw to help people follow.

src/core/SkRecordDraw.h

Index: src/core/SkRecordDraw.h
diff --git a/src/core/SkRecordDraw.h b/src/core/SkRecordDraw.h
index 701e8352c59a1c4b2e2aee12721238aacf424906..f3f00881ea69b2cb1be5892c6935a2b5d16df577 100644
--- a/src/core/SkRecordDraw.h
+++ b/src/core/SkRecordDraw.h
@@ -37,6 +37,9 @@ public:
         : fInitialCTM(initialCTM ? *initialCTM : canvas->getTotalMatrix())
         , fCanvas(canvas) {}
 
+    // This operator calls methods on the |canvas|. The various draw() wrapper
+    // methods around SkCanvas are defined by the DRAW() macro in
+    // SkRecordDraw.cpp.
     template <typename T> void operator()(const T& r) {
         this->draw(r);
     }