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..6816d4ea9bb52a8856c9b9518cfc982787357ee3 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -294,17 +294,22 @@ public:
     SkMatrix        fMatrix;
     int             fDeferredSaveCount;
 
+    // This is the current cumulative depth (aggregate of all done translateZ calls)
+    SkScalar        fCurDrawDepth;
+
     MCRec(bool conservativeRasterClip) : fRasterClip(conservativeRasterClip) {
         fFilter     = nullptr;
         fLayer      = nullptr;
         fTopLayer   = nullptr;
         fMatrix.reset();
         fDeferredSaveCount = 0;
+        fCurDrawDepth      = 0;
 
         // don't bother initializing fNext
         inc_rec();
     }
-    MCRec(const MCRec& prev) : fRasterClip(prev.fRasterClip), fMatrix(prev.fMatrix) {
+    MCRec(const MCRec& prev) : fRasterClip(prev.fRasterClip), fMatrix(prev.fMatrix),
+                               fCurDrawDepth(prev.fCurDrawDepth) {
         fFilter = SkSafeRef(prev.fFilter);
         fLayer = nullptr;
         fTopLayer = prev.fTopLayer;
@@ -1521,6 +1526,16 @@ void SkCanvas::resetMatrix() {
     this->setMatrix(SkMatrix::I());
 }
 
+void SkCanvas::translateZ(SkScalar z) {
+    this->checkForDeferredSave();
+    this->fMCRec->fCurDrawDepth += z;
+    this->didTranslateZ(z);
+}
+
+SkScalar SkCanvas::getZ() const {
+    return this->fMCRec->fCurDrawDepth;
+}
+
 //////////////////////////////////////////////////////////////////////////////
 
 void SkCanvas::clipRect(const SkRect& rect, SkRegion::Op op, bool doAA) {