First pass at Comment API

src/core/SkCanvas.cpp

Index: src/core/SkCanvas.cpp
===================================================================
--- src/core/SkCanvas.cpp	(revision 8806)
+++ src/core/SkCanvas.cpp	(working copy)
@@ -1971,6 +1971,18 @@
     // do nothing. Subclasses may do something with the data
 }
 
+void SkCanvas::beginGroup(const char* description) {
+    // do nothing. Subclasses may do something
+}
+
+void SkCanvas::addComment(const char* kywd, const char* value) {
+    // do nothing. Subclasses may do something
+}
+
+void SkCanvas::endGroup() {
+    // do nothing. Subclasses may do something
+}
+
 //////////////////////////////////////////////////////////////////////////////
 // These methods are NOT virtual, and therefore must call back into virtual
 // methods, rather than actually drawing themselves.