Bug 1227544 - Scaling on 720p devices is broken. r=timdream

Fix the scaling logic for 280-300dpi devices
Some xhdpi devices have real densities as low as 287dpi
but these devices are far too small to have an hdpi UI.

Signed-off-by: Adam Farden <adam@farden.cz>

widget/gonk/nsWindow.cpp

@@ -750,7 +750,7 @@ nsWindow::GetDefaultScaleInternal()
     if (dpi < 200.0) {
         return 1.0; // mdpi devices.
     }
-    if (dpi < 300.0) {
+    if (dpi < 280.0) {
         return 1.5; // hdpi devices.
     }
     // xhdpi devices and beyond.