Added the framework for having canvas/recorder/picture record depth_set's.

src/core/SkRecorder.cpp

Index: src/core/SkRecorder.cpp
diff --git a/src/core/SkRecorder.cpp b/src/core/SkRecorder.cpp
index ff5c27d7e5a6be0fe02f2e343112278c85bb0e0e..042ba64b822914ca0f5c3868b61dab577dd91c5d 100644
--- a/src/core/SkRecorder.cpp
+++ b/src/core/SkRecorder.cpp
@@ -359,6 +359,10 @@ void SkRecorder::didSetMatrix(const SkMatrix& matrix) {
     APPEND(SetMatrix, matrix);
 }
 
+void SkRecorder::didSetZ(const SkScalar z)  {
+    APPEND(SetZ, z);
+}
+
 void SkRecorder::onClipRect(const SkRect& rect, SkRegion::Op op, ClipEdgeStyle edgeStyle) {
     INHERITED(onClipRect, rect, op, edgeStyle);
     SkRecords::RegionOpAndAA opAA(op, kSoft_ClipEdgeStyle == edgeStyle);