Fix a crash in the code which attempts to append the user agent to the list of outgoing headers in ChromeFrame sub requests. The crash occurs because the current headers passed in the BeginningTransaction are NULL.
Fix a crash in the code which attempts to append the user agent to the list of outgoing
headers in ChromeFrame sub requests. The crash occurs because the current headers passed
in the BeginningTransaction are NULL.

Fixes bug http://code.google.com/p/chromium/issues/detail?id=62853

Bug=62853

Review URL: http://codereview.chromium.org/4792002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@65834 0039d316-1c4b-4281-b951-d872f2087c98

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