Add comments to SkRecordDraw to help people follow.

The calls to visit() to execute the SkRecord::Draw::operator() code path
is not clear to read, so adding some comments to help other new-comers
follow this through to the SkCanvas calls.

R=mtklein@chromium.org

Review URL: https://codereview.chromium.org/695403003
diff --git a/src/core/SkRecordDraw.h b/src/core/SkRecordDraw.h
index 701e835..f3f0088 100644
--- a/src/core/SkRecordDraw.h
+++ b/src/core/SkRecordDraw.h
@@ -37,6 +37,9 @@
         : 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);
     }