Fix nits on the advanced VP8 export dialog.

- Correct order of OK and Cancel buttons.
- Correct 2 pass rate controls label.
- Fix alignment of controls/widgets in form.
- Set button type of OK to default (makes <enter> key work).
- Set Cancel button type to cancel (makes <esc> key work).
- Fixes issue http://code.google.com/p/webm/issues/detail?id=239

Change-Id: Ie913bc713bb534914f8f9e2fffbc0fe6f666cc3d