Bug 1402930 - Use the PresContext's override dppx value if set; r=bradwerth

MozReview-Commit-ID: 9DWQdVevIaz

--HG--
extra : rebase_source : 03e184c107e8356c0b8c15a1bfc3d53c092e413a

dom/base/ResponsiveImageSelector.cpp

@@ -373,6 +373,11 @@ ResponsiveImageSelector::SelectImage(bool aReselect)
   }
 
   double displayDensity = pctx->CSSPixelsToDevPixels(1.0f);
+  double overrideDPPX = pctx->GetOverrideDPPX();
+
+  if (overrideDPPX > 0) {
+    displayDensity = overrideDPPX;
+  }
 
   // Per spec, "In a UA-specific manner, choose one image source"
   // - For now, select the lowest density greater than displayDensity, otherwise