Fix cases where we weren't expecting a NULL preference.
Fix cases where we weren't expecting a NULL preference.

This both fixes a specific crash and possibly one or two others that were 
waiting to happen since we weren't always checking for NULL return values from 
PrefService::GetDictionary.


BUG=32079
TEST=(crash should no longer show up)


Review URL: http://codereview.chromium.org/545070

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@36369 0039d316-1c4b-4281-b951-d872f2087c98

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