Add a null pointer check for transaction_ in ContinueDespiteLastError.  The URL request job may have been cancelled but not yet deleted.
Add a null pointer check for transaction_ in
ContinueDespiteLastError.  The URL request job may have
been cancelled but not yet deleted.

R=eroman,jcampan
BUG=6423
Review URL: http://codereview.chromium.org/20146

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@9358 0039d316-1c4b-4281-b951-d872f2087c98

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