[Autofill] Fix crasher introduced in r246814

KeyPressEventCallbacks were not being properly removed and were being called
after the object referenced by the callback had been destroyed.

TBR=estade@chromium.org
BUG=338070

Review URL: https://codereview.chromium.org/147633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247112 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247112