Shrink the minimum height of the message center

New minimum height (77 dip) is matched for a single custom notification.

BUG=642804

Review-Url: https://codereview.chromium.org/2296863004
Cr-Commit-Position: refs/heads/master@{#417473}

ui/message_center/message_center_style.h

@@ -32,7 +32,7 @@ const size_t kMaxVisiblePopupNotifications = 3;
 
 // DIP dimension; H size of the whole card.
 const int kNotificationWidth = 360;
-const int kMinScrollViewHeight = 100;
+const int kMinScrollViewHeight = 77;
 
 // Colors.
 const SkColor kMessageCenterBorderColor = SkColorSetRGB(0xC7, 0xCA, 0xCE);