Add RasterWindowContext_mac

tools/viewer/sk_app/mac/Window_mac.cpp

Index: tools/viewer/sk_app/mac/Window_mac.cpp
diff --git a/tools/viewer/sk_app/mac/Window_mac.cpp b/tools/viewer/sk_app/mac/Window_mac.cpp
index 4aad93c5e382bd2d4b75405ed6dc01e5035728b0..8e707a49c30e8731c497babadb3465a17ca63f09 100644
--- a/tools/viewer/sk_app/mac/Window_mac.cpp
+++ b/tools/viewer/sk_app/mac/Window_mac.cpp
@@ -226,6 +226,10 @@ bool Window_mac::attach(BackendType attachType, const DisplayParams& params) {
     window_context_factory::MacWindowInfo info;
     info.fWindow = fWindow;
     switch (attachType) {
+        case kRaster_BackendType:
+            fWindowContext = NewRasterForMac(info, params);
+            break;
+            
         case kNativeGL_BackendType:
         default:
             fWindowContext = NewGLForMac(info, params);