Fix scrollbar spacing when no scrollbar is visible in dolphin.

Before this commit, Dolphin reserved space for the scrollbar spacing
even when no scrollbar is visible resulting in a ugly gap in the view
when:

1. the theme uses QStyle::SH_ScrollView_FrameOnlyAroundContents and
2. the theme has a positive PM_ScrollView_ScrollBarSpacing.

QtCurve can have both while Oxygen have 1 but not 2.
To reproduce the problem with Oxygen style. Replace the
`width += ....` (which returns -2 or 0 for Oxygen) with `width += 2`.

See more info here:
https://github.com/QtCurve/qtcurve-qt4/issues/9#issuecomment-28630517

CCBUG: 306631
FIXED-IN: 4.11.4
REVIEW: 113902