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..d1c5842ff47bacb061c0ef03571839ec9cc9146a 100644
--- a/include/core/SkCanvas.h
+++ b/include/core/SkCanvas.h
@@ -444,6 +444,13 @@ public:
     */
     void resetMatrix();
 
+    /** Change the current draw depth of the canvas with a translation.

[jvanverth1, 2016/07/13 13:54:24]

Suggestion: Add the specified translation to the current draw depth of the
canvas.
            @param z  The distance to translate in z.
                      Negative into screen, positive out of screen.

            Without translation, the draw depth defaults to 0.

[vjiaoblack, 2016/07/13 14:55:43]

Done.

+        @param z    The SkScalar depth; it's tracked in the save/restore stack.
+                    Negative into screen, positive out of screen.
+                    Defaults to 0.
+    */
+    void translateZ(SkScalar z);
+
     /**
      *  Modify the current clip with the specified rectangle.
      *  @param rect The rect to combine with the current clip
@@ -1245,6 +1252,10 @@ public:
     void temporary_internal_describeTopLayer(SkMatrix* matrix, SkIRect* clip_bounds);
 
 protected:
+    /** Returns the current (cumulative) draw depth of the canvas.
+      */
+    SkScalar getZ() const;
+
     /** After calling saveLayer(), there can be any number of devices that make
         up the top-most drawing area. LayerIter can be used to iterate through
         those devices. Note that the iterator is only valid until the next API
@@ -1307,6 +1318,7 @@ protected:
     virtual void didRestore() {}
     virtual void didConcat(const SkMatrix&) {}
     virtual void didSetMatrix(const SkMatrix&) {}
+    virtual void didTranslateZ(SkScalar) {}
 
     virtual void onDrawAnnotation(const SkRect&, const char key[], SkData* value);
     virtual void onDrawDRRect(const SkRRect&, const SkRRect&, const SkPaint&);