Third small fragment from "Reduce dependence on GrSurface's origin field"

TBR=bsalomon@google.com
Change-Id: Ifae30beb333e9d668fa1378e332e8a4d063a371e
Reviewed-on: https://skia-review.googlesource.com/26348
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/GrBitmapTextureMaker.cpp b/src/gpu/GrBitmapTextureMaker.cpp
index 762b72c..6fb2c6a 100644
--- a/src/gpu/GrBitmapTextureMaker.cpp
+++ b/src/gpu/GrBitmapTextureMaker.cpp
@@ -50,6 +50,7 @@
                                              dstColorSpace);
     }
     if (proxy && fOriginalKey.isValid()) {
+        SkASSERT(proxy->origin() == kTopLeft_GrSurfaceOrigin);
         this->context()->resourceProvider()->assignUniqueKeyToProxy(fOriginalKey, proxy.get());
         // MDB TODO (caching): this has to play nice with the GrSurfaceProxy's caching
         GrInstallBitmapUniqueKeyInvalidator(fOriginalKey, fBitmap.pixelRef());