Add utilities to create GrSurfaceProxy & GrSurfaceContext from backend descs

Change-Id: Iff2278de8ddd6c0dff74e5cf1996702bad31217b
Reviewed-on: https://skia-review.googlesource.com/7647
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/include/private/GrSurfaceProxy.h b/include/private/GrSurfaceProxy.h
index 6f2a751..a08cb26 100644
--- a/include/private/GrSurfaceProxy.h
+++ b/include/private/GrSurfaceProxy.h
@@ -178,6 +178,11 @@
                                               const GrSurfaceDesc&, SkBudgeted,
                                               const void* srcData, size_t rowBytes);
 
+    static sk_sp<GrSurfaceProxy> MakeWrappedBackend(
+                                            GrContext*,
+                                            GrBackendTextureDesc&,
+                                            GrWrapOwnership ownership = kBorrow_GrWrapOwnership);
+
     const GrSurfaceDesc& desc() const { return fDesc; }
 
     GrSurfaceOrigin origin() const {