ChromeOs: Call BrowserList::SessionEnding when shutting down via SIGTERM
ChromeOs: Call BrowserList::SessionEnding when shutting down via SIGTERM

This will make sure exit_cleanly is written.

CleanUP: renamed MarkAsCleanShutdown to MarkAsCleanShutdowInUserProfiles to reflect what the method really does.

BUG=85936
TEST=Along with crosbug.com/16458 fix, ChromeOS's UMA crash number will decrease to the level of
 other platforms.

Review URL: http://codereview.chromium.org/7273038

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@91394 0039d316-1c4b-4281-b951-d872f2087c98

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