[Switch Access] Fix back button visibility

A recent change to TrayBubbleView caused the Switch Access back button
to no longer display. This happened because the back button is
explicitly drawn in OnPaint().

This change restores the previous behavior of the back button.

TBR=jennyz@chromium.org

AX-Relnotes: n/a.
Bug: 1118201
Change-Id: Iae5e43ed1714997b062406a6bc9a8ac0a473bdd5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367787
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#800327}

ash/system/accessibility/switch_access_back_button_bubble_controller.cc

@@ -48,6 +48,7 @@ void SwitchAccessBackButtonBubbleController::ShowBackButton(
     bubble_view_->SetArrow(views::BubbleBorder::BOTTOM_RIGHT);
     bubble_view_->AddChildView(back_button_view_);
     bubble_view_->set_color(SK_ColorTRANSPARENT);
+    bubble_view_->SetPaintToLayer();
     bubble_view_->layer()->SetFillsBoundsOpaquely(false);
 
     widget_ = views::BubbleDialogDelegateView::CreateBubble(bubble_view_);