Add new experimental API to SkPicture to get "id" of current op

src/core/SkPicture.cpp

Index: src/core/SkPicture.cpp
===================================================================
--- src/core/SkPicture.cpp	(revision 13910)
+++ src/core/SkPicture.cpp	(working copy)
@@ -281,6 +281,13 @@
     return OperationList::InvalidList();
 }
 
+size_t SkPicture::EXPERIMENTAL_curOpID() const {
+    if (NULL != fPlayback) {
+        return fPlayback->curOpID();
+    }
+    return 0;
+}
+
 void SkPicture::draw(SkCanvas* surface, SkDrawPictureCallback* callback) {
     this->endRecording();
     if (NULL != fPlayback) {