Views: Add rounded corners support to NativeViewHostMac

This change will enable WebUI bubbles used in top chrome to conform
to the secondary UI spec on Mac platforms.

Before:
https://drive.google.com/file/d/12gCfewhEQrzLAPL2IkCYZy7P_NJNp_jp/view?usp=sharing

After:
https://drive.google.com/file/d/1mkz37-Vmpy6a8HGfvg_JMMiJXdirWtEn/view?usp=sharing

Bug: 1156290
Change-Id: I06b3ef9090cfb8f08c0e9ba095bdeb9807ba107d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577939
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834824}

ui/views/controls/native/native_view_host_mac.mm

@@ -155,8 +155,11 @@ void EnsureNativeViewHasNoChildWidgets(NSView* native_view) {
 
 bool NativeViewHostMac::SetCornerRadii(
     const gfx::RoundedCornersF& corner_radii) {
-  NOTIMPLEMENTED();
-  return false;
+  ui::Layer* layer = GetUiLayer();
+  DCHECK(layer);
+  layer->SetRoundedCornerRadius(corner_radii);
+  layer->SetIsFastRoundedCorner(true);
+  return true;
 }
 
 bool NativeViewHostMac::SetCustomMask(std::unique_ptr<ui::LayerOwner> mask) {