Bug 1516423 - Have WinUnreadBadge own TaskbarWindowPreview. r=mkmelin

In Thunderbird, an instance of `TaskbarWindowPreview` is created when
`WinUnreadBadge.updateUnreadCount` calls `WinTaskbar::GetOverlayIconController`.
However, that instance is freed immdiately after `updateUnreadCount` exits
because nobody owns it.  As a result, the taskbar icon disappears even when
`mail.minimizeToTray = false`.

This patch makes `WinUnreadBadge` own a reference to `TaskbarWindowPreview`
so that its instance is kept alive after the method exits.
This approach is the same as Firefox stores `TaskbarWindowPreview` as
`DownloadsTaskbar._taskbarProgress`.

Differential Revision: https://phabricator.services.mozilla.com/D110191

--HG--
extra : amend_source : 6817b4fb4e049a3e5005505fbac5eae9994a404c

mailnews/base/src/WinUnreadBadge.jsm

@@ -184,6 +184,12 @@ function downsampleBy4X(window, canvas) {
  * A module to manage the unread badge icon on Windows.
  */
 var WinUnreadBadge = {
+  /**
+   * Keeping an instance of nsITaskbarOverlayIconController alive
+   * to show a taskbar icon after the updateUnreadCount method exits.
+   */
+  _controller: null,
+
   /**
    * Update the unread badge.
    * @param {number} unreadCount - Unread message count.
@@ -194,10 +200,12 @@ var WinUnreadBadge = {
     if (!window) {
       return;
     }
-    let controller = taskbar.getOverlayIconController(window.docShell);
+    if (!this._controller) {
+      this._controller = taskbar.getOverlayIconController(window.docShell);
+    }
     if (unreadCount == 0) {
       // Remove the badge if no unread.
-      controller.setOverlayIcon(null, "");
+      this._controller.setOverlayIcon(null, "");
       return;
     }
 
@@ -228,6 +236,6 @@ var WinUnreadBadge = {
     // setOverlayIcon.
     await new Promise(resolve => window.setTimeout(resolve));
 
-    controller.setOverlayIcon(icon, unreadTooltip);
+    this._controller.setOverlayIcon(icon, unreadTooltip);
   },
 };