Fixing Incognito Mode crash when visiting chrome://{thumb,thumb2,thumbnails}

In Incognito Mode, thumbnail services is NULL, causing chrome://{thumb,thumb2,thumbnails} (and also chrome-search://) to crash.

We now do what chrome://bookmarks does: launch Chrome in default profile to open the above links.

BUG=307715

Review URL: https://codereview.chromium.org/27069009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230137 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230137