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: http://src.chromium.org/svn/trunk/src@39757 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: e424fb583066bca7c2f3b0af6f7aaec28e3526e9
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser