Remove gray line under search box

Changes:
Draw the separator between search box and search results only when
parent view's height is larger than search box height.

BUG=750951

Change-Id: I5ddd8b607c91b8a2272174157f0d232f18a0ee69
Reviewed-on: https://chromium-review.googlesource.com/595245Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491614}

ui/app_list/views/search_result_page_view.cc

@@ -98,6 +98,8 @@ class SearchResultPageBackground : public views::Background {
     flags.setColor(color_);
     canvas->DrawRoundRect(bounds, corner_radius_, flags);
 
+    if (bounds.height() <= kSearchBoxHeight)
+      return;
     // Draw a separator between SearchBoxView and SearchResultPageView.
     bounds.set_y(kSearchBoxHeight);
     bounds.set_height(kSeparatorThickness);