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

src/core/SkCanvas.cpp

Index: src/core/SkCanvas.cpp
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index ce90275047a6466cf9217e32bc0e9505ed8eff36..2be14f225e1c28cd21dd838d78388959ddd22846 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -294,6 +294,8 @@ public:
     SkMatrix        fMatrix;
     int             fDeferredSaveCount;
 
+    SkScalar        fCurDrawDepth;
+
     MCRec(bool conservativeRasterClip) : fRasterClip(conservativeRasterClip) {
         fFilter     = nullptr;
         fLayer      = nullptr;
@@ -1521,6 +1523,17 @@ void SkCanvas::resetMatrix() {
     this->setMatrix(SkMatrix::I());
 }
 
+void SkCanvas::setZ(SkScalar z) {
+    this->checkForDeferredSave();
+    this->fMCRec->fCurDrawDepth = z;
+    this->didSetZ(z);
+}
+
+SkScalar SkCanvas::getZ() const {
+    return this->fMCRec->fCurDrawDepth;
+}
+
+
 //////////////////////////////////////////////////////////////////////////////
 
 void SkCanvas::clipRect(const SkRect& rect, SkRegion::Op op, bool doAA) {