Allow extensions to remove and override the bookmark shortcut key

This feature is enabled for dev behind the
--enable-override-bookmarks-ui=1 feature flag, and for all releases for
internal bookmarks extensions.

Implements the shortcut key aspect of the
Remove Bookmark Shortcut Chrome API proposal:
https://docs.google.com/a/chromium.org/document/d/1C2Mle92O9uGlji5y5gGDM5tNJ_tVE1Vb-2xgsZPNDTk

BUG=335655
R=erg@chromium.org, finnur@chromium.org, shess@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/143493005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251368 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251368