Revert 233619 "Fix crash on login, don't pass PrepareKeyContext ..."

Compile failure on Linux Chromium OS ASAN Builder

> 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

TBR=nkostylev@chromium.org

Review URL: https://codereview.chromium.org/64033003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233621 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233621