[Gtk] on middle click, open all bookmarks in tabs
[Gtk] on middle click, open all bookmarks in tabs

When a subfolder (i.e. a folder in a dropdown menu, not on the bookmark bar itself) is middle clicked, it should open all sub nodes in tabs.

BUG=34416
TEST=none

Review URL: http://codereview.chromium.org/2968004

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@52123 0039d316-1c4b-4281-b951-d872f2087c98

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