ENYO-388: Added explicit dimensions for button with image in sample.

### Issue
The button with image in the Enyo `ButtonSample` was not appearing with the full height on iOS devices. This appears to be an implementation detail in iOS Safari (and UIWebView i.e. iOS Chrome) when placing an image inside a button tag.

### Fix
Short of rewriting this sample, we set explicit dimensions for this control.

### Notes
This also needs to be cherry-picked into the `2.5.1-release` branch.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>