[Polymer Designer] A workaround to fix the disappearing elements palette

TBR

Fixes #3634 (or rather patches it).

There is some issue probably with WebKit or `<webview>` failing to render stacking contexts when the CSS is changing or something like that. The palette's opacity was set to 0.9, and that somehow caused it to disappear (actually, just fail to render) when any category was expanded with a mouse click. Setting opacity to 1 (and thus eliminating a stacking context) somehow fixed it.

There is already [a somewhat related place](https://github.com/dart-lang/chromedeveditor/blob/master/cde-polymer-designer.dart/lib/cde_polymer_designer.js#L203) in `cde-polymer-designer.dart`.