Use zero height / width for compute canvas (#3369)

* Make the blockly compute canvas have no height or width

core/css.js

@@ -772,6 +772,12 @@ Blockly.Css.CONTENT = [
   '.blocklyDropDownDiv .goog-menuitem-rtl .goog-menuitem-icon {',
     'float: right;',
     'margin-right: -24px;',
-  '}'
+  '}',
+
+  '.blocklyComputeCanvas {',
+    'position: absolute;',
+    'width: 0;',
+    'height: 0;',
+  '}',
   /* eslint-enable indent */
 ];