Fix scrollbar spacing when no scrollbar is visible in dolphin.

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