Bug 1381097: Part 1 - Add default-false pref to enable APZ in remote popups. r=kats

Ideally, we definitely want APZ in remote popup browsers for the sake of
responsiveness. However, there are currently some serious painting and
checkerboarding issues that make APZ popups unsuitable for release
populations.

Once these issues are addressed, we should enable the preference by default.
But given the relative lack of testing, and the issues we've seen so far, we
should keep the preference so that we can disable it with a hotfix if further
issues arise.

MozReview-Commit-ID: GOZRdsmLNZR

widget/nsBaseWidget.cpp

@@ -897,7 +897,8 @@ nsBaseWidget::UseAPZ()
   return (gfxPlatform::AsyncPanZoomEnabled() &&
           (WindowType() == eWindowType_toplevel ||
            WindowType() == eWindowType_child ||
-           (WindowType() == eWindowType_popup && HasRemoteContent())));
+           (WindowType() == eWindowType_popup && HasRemoteContent() &&
+            gfxPrefs::APZPopupsEnabled())));
 }
 
 void nsBaseWidget::CreateCompositor()