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

include/core/SkCanvas.h

Index: include/core/SkCanvas.h
diff --git a/include/core/SkCanvas.h b/include/core/SkCanvas.h
index ec14829ca85296d12d79d27a04073c6ee587c20e..3d75f8c04972d561ce492306a1cdff5cfee70e35 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -444,6 +444,17 @@ public:
     */
     void resetMatrix();
 
+    /** Set the current draw depth of the canvas.

[reed1, 2016/07/11 19:36:28]

Is the default 0 or 1?

[vjiaoblack, 2016/07/11 19:48:13]

Done.

+        @param z    The SkScalar depth; it's tracked in the save/restore stack.
+                    Negative into screen, positive out of screen.
+
+    */
+    void setZ(SkScalar z);
+
+    /** Returns the current draw depth of the canvas.
+    */
+    SkScalar getZ() const;
+
     /**
      *  Modify the current clip with the specified rectangle.
      *  @param rect The rect to combine with the current clip
@@ -1307,6 +1318,7 @@ protected:
     virtual void didRestore() {}
     virtual void didConcat(const SkMatrix&) {}
     virtual void didSetMatrix(const SkMatrix&) {}
+    virtual void didSetZ(SkScalar) {}
 
     virtual void onDrawAnnotation(const SkRect&, const char key[], SkData* value);
     virtual void onDrawDRRect(const SkRRect&, const SkRRect&, const SkPaint&);