Fix opera bookmarks

[Phoenix-]

Fix for #2598

Problem Description

Bookmarks from Opera browser are not being indexed unless placed onto bookmarks toolbar.
Checked for Opera 107.0.5045.36 but I dont think its linked to any particular version.
Bookmarks file for opera differs from Chrome one, note 'custom_root' sub-object

{
   "roots": {
      "bookmark_bar": {
         "children": [  ]
      },
      "custom_root": {
         "pinboard": {
            "children": [  ]
         },
         "speedDial": {
            "children": [ 
               {
                  "children": [  ]
               } 
            ]
         },
         "trash": {
            "children": [  ]
         },
         "unsorted": {
            "children": [  ]
         },
         "unsyncedPinboard": {
            "children": [  ]
         },
         "userRoot": {
            "children": [ {
               "children": [ {
                  "name": "...",
                  "type": "url",
                  "url": "https://..."
               } ],
               "name": "...",
               "type": "folder"
            },
<...>

Current implementation of EnumerateFolderBookmark method skips any object without 'children' field.
My fix slightly changes EnumerateRoot method to check for possible 'custom_root' field and if found treat it as a root itself.
Should not break other browsers and fixes Opera.

This fix takes "custom_root" sub-object into account.

[taooceros]

interesting, will any other browser being affected? I feel like it is weird to modify the chromiumloader

[Phoenix-]

Nothing else should be affected. The behavior changes only when tag with name 'custom_root' is encountered,

[Phoenix-]

Another way exists to fix the issue. I can write another implementation of IBookmarkLoader for Opera. But it will differ from Chromium one by literally 2 lines of code. Not sure it is a good idea.

[taooceros]

I think it's ok for now. Could you add a comment for that? If we have more exception then we consider more abstraction.

[Phoenix-]

Added comment with details.

[VictoriousRaptor]

LGTM. BTW could you please copy the problem description part of #2598 to this PR?

[Phoenix-]

Added description