Fix hidden shelf reveal on mouse hover

On some scale factors the bottom most pixel is outside of
the display bounds. Snapping shelf region to display edge fixes
the problem.

Bug: 899007
Test: manual
Change-Id: I1eeb342756b46e683d0f30f96f1c428fbb74d7ca
Reviewed-on: https://chromium-review.googlesource.com/c/1320013Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605748}

ash/shelf/shelf_layout_manager.cc

@@ -942,7 +942,8 @@ gfx::Rect ShelfLayoutManager::GetVisibleShelfBounds() const {
   gfx::Rect shelf_region = shelf_widget_->GetWindowBoundsInScreen();
   DCHECK(!display_.bounds().IsEmpty());
   shelf_region.Intersect(display_.bounds());
-  return shelf_region;
+  return screen_util::SnapBoundsToDisplayEdge(shelf_region,
+                                              shelf_widget_->GetNativeWindow());
 }
 
 gfx::Rect ShelfLayoutManager::GetAutoHideShowShelfRegionInScreen() const {