keyboard: Make sure the event handlers are destroyed/removed correctly.

BUG=none
R=kevers@chromium.org

Review URL: https://codereview.chromium.org/132423003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244942 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244942