Fix for y flip on hit-test ray input

Bug: 848959
Change-Id: I99c958ba7ae1de5c233e77d085dbd20880baf6ca
Reviewed-on: https://chromium-review.googlesource.com/1089812
Commit-Queue: Max Rebuschatis <lincolnfrog@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565695}

chrome/browser/android/vr/arcore_device/arcore_impl.cc

@@ -318,7 +318,11 @@ bool ARCoreImpl::TransformRayToScreenSpace(const mojom::XRRayPtr& ray,
   }
 
   screen_point->set_x((screen_point_3d.x() + 1) / 2);
-  screen_point->set_y((screen_point_3d.y() + 1) / 2);
+  // The calculated point in GL's normalized device coordinates (NDC) ranges
+  // from -1..1, with -1, -1 at the bottom left of the screen, +1 at the top.
+  // The output screen space coordinates range from 0..1, with 0, 0 at the
+  // top left.
+  screen_point->set_y((-screen_point_3d.y() + 1) / 2);
   return true;
 }