Fixing M33 and M34 stable blocker crash with InsertAppLauncherItem

The problem appears to be that the function GetLastActiveWebContents returns a WebContents pointer to some content which got already deleted. (See comment from CL).

Note that this is only used in InsertAppLauncherItem to find the activity state. Note that this indicates that we could even get rid of that map and use app_id_to_web_contents to find if a tab is currently active. But again - keeping the CL small for the backport.

This seems to be the most secure and isolated for for M33 and 34 - I will create an issue to follow up on this the proper way.

BUG=341250
TEST=none (no test case known)

Review URL: https://codereview.chromium.org/160803002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250874 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250874