Fix a ChromeFrame crash seen in the Milestone 10 stable channel. The crash occurs in the external tab container while running unload handlers on the page, specifically when attempting to send a reply back to IE because the reply message is NULL. It is not clear how the reply message is NULL. 
Fix a ChromeFrame crash seen in the Milestone 10 stable channel. The crash occurs in the external
tab container while running unload handlers on the page, specifically when attempting to
send a reply back to IE because the reply message is NULL. It is not clear how the reply message
is NULL. 

Added a NULL check at the corresponding location with a NOTREACHED to help catch this in debug
builds. Also set the reply message member variable before invoking the browser function to
run the unload handlers as a crash could occur if the before unload handler notifications arrive
in this context. Does not seem to be the case here though.

Fixes bug http://code.google.com/p/chromium/issues/detail?id=76998

BUG=76998
R=amit
Review URL: http://codereview.chromium.org/6708077

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@79017 0039d316-1c4b-4281-b951-d872f2087c98

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