Build: Report compressed sizes in compare_size

Build: Report compressed sizes in compare_size

Just like it has always worked in Core. This will help with size comparisons
between 1.13 & 1.14.

After this PR:

```
$ grunt sizer
Running "requirejs:js" (requirejs) task

Running "uglify:main" (uglify) task
>> 1 file created 549 kB → 267 kB

Running "compare_size:all" (compare_size) task
   raw     gz Sizes
549319 128757 dist/jquery-ui.js
266710  69612 dist/jquery-ui.min.js

Done.
```

Before, only the less interesting `raw` sizes were reported.

Closes gh-2254
Ref gh-2248