Fix NULL pointer crash triggered by ResetProfileSettingsHandler::InitializePage().

Added nullity checks around usages of |automatic_profile_resetter_| introduced by r240179 in ResetProfileSettingsHandler, as this pointer may be NULL when signed in as guest.

BUG=330337
R=engedy@chromium.org
TBR=bauerb@chromium.org
TEST=verify that settings page no longer crashes when signed in as guest.

Review URL: https://codereview.chromium.org/120043004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242295 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242295