[RootLayerScrolls] Fix logic for computing screen distance of iframe

BUG=823365
R=skobes@chromium.org,sclittle@chromium.org

Change-Id: I400bae2f5a35075b06e42a2b2796772c3a0a687f
Reviewed-on: https://chromium-review.googlesource.com/1107164Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568878}

third_party/blink/renderer/core/frame/local_frame_view.cc

@@ -4559,20 +4559,17 @@ void LocalFrameView::RecordDeferredLoadingStats() {
     return;
   }
 
-  IntRect parent_rect = parent->FrameRect();
+  IntSize parent_size(parent->Size());
   // First clause: for this rough data collection we assume the user never
   // scrolls right.
-  if (frame_rect.X() >= parent_rect.Width() || parent_rect.Height() <= 0)
+  if (frame_rect.X() >= parent_size.Width() || parent_size.Height() <= 0)
     return;
 
   int this_frame_screens_away = 0;
   // If an frame is created above the current scoll position, this logic counts
   // it as visible.
-  if (frame_rect.Y() > parent->GetScrollOffset().Height()) {
-    this_frame_screens_away =
-        (frame_rect.Y() - parent->GetScrollOffset().Height()) /
-        parent_rect.Height();
-  }
+  if (frame_rect.Y() > 0)
+    this_frame_screens_away = frame_rect.Y() / parent_size.Height();
   DCHECK_GE(this_frame_screens_away, 0);
 
   int parent_screens_away = 0;