Use SMALL_SHADOW for omnibox popup

RoundedOmniboxResultsFrame is the content view for omnibox's
autocomplete popup widget. It is in a TYPE_POPUP widget. For this kind
of widget, SMALL_SHADOW and BIG_SHADOW has no difference. Since
SMALL_SHADOW is the default value for most dialogs, use this type
instead. We will eliminate the BIG_SHADOW eventually.

Bug: 940027
Change-Id: I4da68c4eeb9eeecb44b0a1a9f6bf610880b90acf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612150Reviewed-by: Tommy Li <tommycli@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840660}

chrome/browser/ui/views/omnibox/rounded_omnibox_results_frame.cc

@@ -166,8 +166,8 @@ RoundedOmniboxResultsFrame::RoundedOmniboxResultsFrame(
 
   // Initialize the shadow.
   auto border = std::make_unique<views::BubbleBorder>(
-      views::BubbleBorder::Arrow::NONE, views::BubbleBorder::Shadow::BIG_SHADOW,
-      gfx::kPlaceholderColor);
+      views::BubbleBorder::Arrow::NONE,
+      views::BubbleBorder::Shadow::SMALL_SHADOW, gfx::kPlaceholderColor);
   border->SetCornerRadius(corner_radius);
   border->set_md_shadow_elevation(kElevation);
   SetBorder(std::move(border));