Fix crash on login, don't pass PrepareKeyContext members by reference

BUG=315624
NOTRY=true
TEST=on a real device, crash is gone.

Review URL: https://codereview.chromium.org/64213003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233619 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233619