[MdTextButton] Apply rounded rectangle to background painting

In the final OnPaintBackground() method, it currently uses a standard
rectangle to "dim" the background on hover (or animation). This
resulted in some dimming outside a rounded button. This change uses a
rounded rectangle instead, using the existing corner radius.

Bug: 866136
Change-Id: Ifb8ad476e231049efbf73e1dc64c0c1f0f113897
Reviewed-on: https://chromium-review.googlesource.com/1156877
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579819}

ui/views/controls/button/md_text_button.cc

@@ -106,7 +106,11 @@ void MdTextButton::OnPaintBackground(gfx::Canvas* canvas) {
   if (hover_animation().is_animating() || state() == STATE_HOVERED) {
     const int kHoverAlpha = is_prominent_ ? 0x0c : 0x05;
     SkScalar alpha = hover_animation().CurrentValueBetween(0, kHoverAlpha);
-    canvas->FillRect(GetLocalBounds(), SkColorSetA(SK_ColorBLACK, alpha));
+    cc::PaintFlags flags;
+    flags.setColor(SkColorSetA(SK_ColorBLACK, alpha));
+    flags.setStyle(cc::PaintFlags::kFill_Style);
+    flags.setAntiAlias(true);
+    canvas->DrawRoundRect(gfx::RectF(GetLocalBounds()), corner_radius_, flags);
   }
 }