Widget opacity goes from 0 to 1, not 0 to 255.

chrome/browser/ui/views/new_back_shortcut_bubble.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/views/new_back_shortcut_bubble.h"
 
 #include <utility>
 
 #include "base/message_loop/message_loop.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 67 matching lines @@
   animation_->Show();
 
   // Wait a few seconds before hiding.
   hide_timeout_.Start(FROM_HERE,
                       base::TimeDelta::FromMilliseconds(kShowDurationMs), this,
                       &NewBackShortcutBubble::OnTimerElapsed);
 }
 
 void NewBackShortcutBubble::AnimationProgressed(
     const gfx::Animation* animation) {
-  int opacity = animation_->CurrentValueBetween(0, 255);
+  float opacity = static_cast<float>(animation_->CurrentValueBetween(0.0, 1.0));
   if (opacity == 0) {
     popup_->Hide();
   } else {
     if (!popup_->IsVisible())
       popup_->Show();
 
     popup_->SetOpacity(opacity);
   }
 }
 
@@ skipping 10 matching lines @@
   int desired_top = kPopupTopPx - view_->border()->GetInsets().top();
   int y = widget_bounds.y() + desired_top;
   return gfx::Rect(gfx::Point(x, y), size);
 }
 
 void NewBackShortcutBubble::OnTimerElapsed() {
   // Hide the bubble.
   animation_->SetSlideDuration(kSlideOutDurationMs);
   animation_->Hide();
 }