Fix devicePixelRatio getter for embedded QWindows.

m_nsWindow is not set for non-toplevel QWindows,
causing devicePixelRatio to always return 1.

Use [m_contentView window] instead.

Change-Id: I6689a70812c9484f103b5e706fe4c1b76406b750
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -837,11 +837,9 @@ QCocoaMenuBar *QCocoaWindow::menubar() const
 
 qreal QCocoaWindow::devicePixelRatio() const
 {
-    if (!m_nsWindow)
-        return 1.0;
 #if MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_7
     if (QSysInfo::MacintoshVersion >= QSysInfo::MV_10_7) {
-        return qreal([m_nsWindow backingScaleFactor]);
+        return qreal([[m_contentView window] backingScaleFactor]);
     } else
 #endif
     {