add saveBehind to canvas

Bug: skia:
Change-Id: I62e48b116b2d7efef585d4f465441f24ae4d2b29
Reviewed-on: https://skia-review.googlesource.com/c/177892
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/tools/debugger/SkDebugCanvas.cpp b/tools/debugger/SkDebugCanvas.cpp
index 450af1a..9d018f1 100644
--- a/tools/debugger/SkDebugCanvas.cpp
+++ b/tools/debugger/SkDebugCanvas.cpp
@@ -467,6 +467,11 @@
     return kNoLayer_SaveLayerStrategy;
 }
 
+bool SkDebugCanvas::onDoSaveBehind(const SkRect* subset) {
+    // TODO
+    return false;
+}
+
 void SkDebugCanvas::didSetMatrix(const SkMatrix& matrix) {
     this->addDrawCommand(new SkSetMatrixCommand(matrix));
     this->INHERITED::didSetMatrix(matrix);