Allow setting devicePixelRatio

[DeviaVir]

As requested, reviving vitallium/qtwebkit#5 on this repo.

Original PR text:

Attempting to revive ariya/phantomjs#12839 upstream, this should allow webpage.ccp next to make use of this and actually set and overwrite the pixelratio.

Refer to ariya/phantomjs#10964 for the what and why.

[DeviaVir]

ping @annulen, could you please review these changes? They've been in flight at least a year now.

[annulen]

I'm not sure this is a correct way to do things. If page has a view, it will also call setDevicePixelRatio from QWebPagePrivate::_q_updateScreen, resulting in inconsistent behavior. Also, there is unfixed crash #461, I think it would be better to fix it first before exposing on more way to trigger it.