The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/63a16add8253bc6a138329c648ba82a5492f11f0

commit 63a16add8253bc6a138329c648ba82a5492f11f0
Author: fukino <fukino@chromium.org>
Date: Mon Jun 12 10:56:17 2017

Notification: Implement two-leveled notification.

The new-style notification should have two states: Collapsed(default) and Expanded.
To implement the feature, this CL does:
* Create NotificationHeaderView which is responsible to keeps app_icon, app_name, expand_button, settings_button, and close_button.
* When the expand_button is clicked, some view's visibility and message_view's line limit is modified to have "expanded" view.

For simplicity, I avoided dynamic creation/deletion of views on expanded state change. The expanded state only changes the existing view's property.
If it hit noticeable performance regression, I'll update the way to change the view.

BUG= 728500 , 726242 
TEST=manually tested expand/collapse function on following notifications using --enabled-new-style-notification
* A basic notification with one-line message (This doesn't have expand icon.)
* A basic notificatoin with two-line message
* A basic notification with two buttons
* A image notification

Review-Url: https://codereview.chromium.org/2925263003
Cr-Commit-Position: refs/heads/master@{#478587}

[modify] https://crrev.com/63a16add8253bc6a138329c648ba82a5492f11f0/ui/message_center/BUILD.gn
[add] https://crrev.com/63a16add8253bc6a138329c648ba82a5492f11f0/ui/message_center/vector_icons/notification_expand_less.icon
[add] https://crrev.com/63a16add8253bc6a138329c648ba82a5492f11f0/ui/message_center/vector_icons/notification_expand_more.icon
[add] https://crrev.com/63a16add8253bc6a138329c648ba82a5492f11f0/ui/message_center/views/notification_header_view.cc
[add] https://crrev.com/63a16add8253bc6a138329c648ba82a5492f11f0/ui/message_center/views/notification_header_view.h
[modify] https://crrev.com/63a16add8253bc6a138329c648ba82a5492f11f0/ui/message_center/views/notification_view_md.cc
[modify] https://crrev.com/63a16add8253bc6a138329c648ba82a5492f11f0/ui/message_center/views/notification_view_md.h

Thanks fukino@!