Update CodeMirror for sprint 32. Avoid hang by setting width on mock editor holder in unit tests.

@RaymondLim 

See comments in #5179. I verified that all the automated tests pass and smoke tests are ok after this change.

Could you also verify that the issues caused by https://github.com/marijnh/CodeMirror/issues/1787 are also fixed after this update, and close that bug as well as our own related issues? Thanks.