Propogate WARN_UNUSED_RESULT up from PickeIterator methods to Pickle wrapper methods.

I noticed this while investigating an unrelated bug which might have been caused
by a failure to check return codes whilst unpickling. So make sure this always
happens.

No new bugs kicked loose, the other changes just tidy up a few places where
we'd get the error otherwise.

R=dcheng@chromium.org, jam@chromium.org, jar@chromium.org

Review URL: https://codereview.chromium.org/26997006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228945 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228945