Make sk_app::WindowContext directly create a SkSurface without an intermediate GrRenderTarget
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2165703002

Review-Url: https://codereview.chromium.org/2165703002
diff --git a/tools/viewer/sk_app/GLWindowContext.cpp b/tools/viewer/sk_app/GLWindowContext.cpp
index b960da4..31ba01c 100644
--- a/tools/viewer/sk_app/GLWindowContext.cpp
+++ b/tools/viewer/sk_app/GLWindowContext.cpp
@@ -83,9 +83,8 @@
             GrGLint buffer;
             GR_GL_CALL(fBackendContext, GetIntegerv(GR_GL_FRAMEBUFFER_BINDING, &buffer));
             desc.fRenderTargetHandle = buffer;
-            fRenderTarget.reset(fContext->textureProvider()->wrapBackendRenderTarget(desc));
 
-            fSurface = this->createRenderSurface(fRenderTarget, fActualColorBits);
+            fSurface = this->createRenderSurface(desc, fActualColorBits);
         }
     }