theme: Unbind Ctrl-Shift-Backspace in GtkUndoView.
I'm changing Chrome's "Clear Browsing Data" accelerator to
Ctrl-Shift-Backspace, so this change tells GTK+ to unbind
that instead of Ctrl-Shift-Delete.
This change makes the "Clear Browsing Data" accelerator work
when the focus is in the omnibox, while still honoring the
"delete to the beginning of the line" accelerator when the
focus is in e.g. a textarea.
TEST=manual: check that the dialog is opened when i hit ctrl-shift-backspace while the focus is in the omnibox or in a non-editable part of a web page, and that the text-editing accelerator works when the focus is in a textarea