Moved code that generates APP_TERMINATING notification into BrowserList so it can be used on all platforms. Updated observers (ExtensionProcessManager,
BackgroundContents) to listen for APP_TERMINATING instead of listening for
BROWSER_CLOSED on some platforms.

APP_TERMINATING is now sent just before the main message loop exits rather than  just after, but no code depends on this timing.

Updated Mac code to always call BrowserList::CloseAllBrowsers() even if there
are no open browsers, to ensure that APP_TERMINATING is always fired.

Changed BackgroundContentsService to keep the browser process alive when there
are BackgroundContents running and updated the unit tests.

Renamed BrowserList::IsInPersistentMode() => WillShutdownWhenLastBrowserCloses() and AllBrowsersClosed() => AllBrowsersClosedAndAppExiting() to more precisely indicate their true functions.

Exposed BrowserProcess::ModuleRefCount() so BrowserList can determine when the application is going to exit so the right notifications/callbacks can be generated.

Updated background-auto-update-restart code to use new BrowserList APIs to determine whether the application is running "in the background".

Added code to cancel shutdown on all plaforms if the user selects cancel in an onbeforeunload dialog.
BUG=45275
TEST=RunInBackgroundTest (ui_test)

Review URL: http://codereview.chromium.org/2866034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@53092 0039d316-1c4b-4281-b951-d872f2087c98