Only set both height and width of editor when shadow DOM is used

These work inside a shadow root, but break the flexbox behavior when
the shadow DOM is disabled.

static/text-editor-shadow.less

@@ -2,7 +2,11 @@
 @import "octicon-utf-codes";
 @import "octicon-mixins";
 
-.editor, .editor-contents {
+.editor-contents {
+  width: 100%;
+}
+
+.editor.shadow, .editor.shadow .editor-contents {
   height: 100%;
   width: 100%;
 }