Made ShuttingDownWithoutCloseBrowsers a explicit flag.
 Use END_SESSION shutdown type if chrome recieves SIGTERM AND there are tabs that may
 block shutdown.
 Make sure APP_TERMINATING is sent only once.

This fixes SIGABORT crash in two shutdown scenarios:
1) powering off when chrome has beforeunload handler, or
downloads in progress.
2) singout from screen locker when chrome has beforeunload handler or downloads in progress.

 This is simple version of fix to merge to release branch. I'm working on
another CL that will (hopefully) cleanup a bit more.

BUG=chromium-os:20460
TEST=see bug for repro step


Review URL: http://codereview.chromium.org/8262009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@106988 0039d316-1c4b-4281-b951-d872f2087c98