Bug 957884: Add return statement to default case in SurfaceFormatToDXGIFormat(), to address build warning. r=nical

gfx/layers/d3d11/TextureD3D11.cpp

@@ -32,6 +32,7 @@ SurfaceFormatToDXGIFormat(gfx::SurfaceFormat aFormat)
       return DXGI_FORMAT_A8_UNORM;
     default:
       MOZ_ASSERT(false, "unsupported format");
+      return DXGI_FORMAT_UNKNOWN;
   }
 }