dark_mode: kOpaque instead of kTransparent80.

Update on the spec.

Bug: 1128755
Change-Id: I62a9cf60f33f3bc9c162cdb1aad68a811a2c4e6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464046Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815840}

ash/wm/gestures/back_gesture/back_gesture_contextual_nudge.cc

@@ -277,8 +277,7 @@ class BackGestureContextualNudge::ContextualNudgeView
       round_rect_flags.setStyle(cc::PaintFlags::kFill_Style);
       round_rect_flags.setAntiAlias(true);
       round_rect_flags.setColor(DeprecatedGetBaseLayerColor(
-          AshColorProvider::BaseLayerType::kTransparent80,
-          kLabelBackgroundColor));
+          AshColorProvider::BaseLayerType::kOpaque, kLabelBackgroundColor));
       gfx::Rect label_bounds(label_->GetMirroredBounds());
       label_bounds.Inset(/*horizontal=*/-kLabelCornerRadius,
                          /*vertical=*/-kLabelTopBottomInset);