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..ce9e52adef727414871f147c50e085c3b98c4f1e 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -444,6 +444,19 @@ public:
     */
     void resetMatrix();
 
+    /** Set the current draw depth of the canvas.
+        @param z    The integer depth, intended between 0 - 255 inclusive
+                    0 is lowest (into screen), 255 is highest (out of screen)
+    */
+    void setZ(uint32_t z) {
+        this->fCurDrawDepth = z;
+        this->didSetZ(z);
+    }
+
+    /** Returns the current draw depth of the canvas.
+    */
+    uint32_t getZ() const { return this->fCurDrawDepth; }
+
     /**
      *  Modify the current clip with the specified rectangle.
      *  @param rect The rect to combine with the current clip
@@ -1307,6 +1320,7 @@ protected:
     virtual void didRestore() {}
     virtual void didConcat(const SkMatrix&) {}
     virtual void didSetMatrix(const SkMatrix&) {}
+    virtual void didSetZ(const uint32_t) {}
 
     virtual void onDrawAnnotation(const SkRect&, const char key[], SkData* value);
     virtual void onDrawDRRect(const SkRRect&, const SkRRect&, const SkPaint&);
@@ -1385,6 +1399,8 @@ protected:
                         const SkImageFilter* imageFilter = NULL);
 
 private:
+    uint32_t fCurDrawDepth = 0;
+
     static bool BoundsAffectsClip(SaveLayerFlags);
     static SaveLayerFlags LegacySaveFlagsToSaveLayerFlags(uint32_t legacySaveFlags);