Adds code to unload digital persona

Digital persona hooks us in bad ways, leading to obscure crashes. In
particular their hook procedures trigger sending messages, leading to
states we are not set up to handle, nor should we have to handle.

BUG=291265
TEST=none
R=cpu@chromium.org

Review URL: https://codereview.chromium.org/26541003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227684 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227684