Force no default focus ring for assistant button

Bug: 1025363
Change-Id: Ief058d765aead7ea71f9b866a53165bd99a4c991
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919800Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716447}

ash/assistant/ui/base/assistant_button.cc

@@ -29,6 +29,10 @@ AssistantButton::AssistantButton(views::ButtonListener* listener,
   constexpr SkColor kInkDropBaseColor = SK_ColorBLACK;
   constexpr float kInkDropVisibleOpacity = 0.06f;
 
+  // Avoid drawing default focus rings since assistant buttons use
+  // a custom highlight on focus.
+  SetInstallFocusRingOnFocus(false);
+
   // Focus.
   SetFocusForPlatform();