Speculatve fix for a chrome crash caused in a chrome frame instance while processing an automation message to focus through an external tab. Based on the crash dump, it appears that the tab_contents_ member is destroyed and set to NULL in the context of a call to TabContents::Focus which calls the Windows API SetFocus on the native HWND, which could dispatch messages like WM_DESTROY for the external tab.
Speculatve fix for a chrome crash caused in a chrome frame instance while processing an automation message
to focus through an external tab. Based on the crash dump, it appears that the tab_contents_ member is destroyed
and set to NULL in the context of a call to TabContents::Focus which calls the Windows API SetFocus on the native
HWND, which could dispatch messages like WM_DESTROY for the external tab.

Fix is to add a second NULL check for tab_contents_ after the call to TabContents::Focus. Added a comment
describing this scenario in the code.

Fixes bug http://code.google.com/p/chromium/issues/detail?id=29246

Bug=29246

Review URL: http://codereview.chromium.org/653005

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@39757 0039d316-1c4b-4281-b951-d872f2087c98

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