The vim mode changed the way the keymap is implemented. In CM4 vim
mode now uses the call method and does not defined specific keys
within the map. Copying the map object and deleting entries was a
no-op and that meant vim mode still handled keys like Ctrl-C that
Gerrit wanted to propagate up into the browser.
Build our own KeyMap object that sets up non-bindings for the
specific keys that should go up the DOM tree, and then fallback
to the KeyMap object provided by vim mode.
Change-Id: I3aa14a170cedf6100a8de0df88a584810980530b
04303d53df