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The motivation for this is that we may one day want to make background mode be
the default, which this flag currently achieves, but in that case we'd need to
respect the user's desire to disable background mode altogether.

TEST=Starting chrome with --keep-alive-for-test should display a chrome icon in the system tray unless background mode is disabled from the advanced tab in chrome://settings. Toggling that setting should toggle the presence of the icon.

Review URL: https://chromiumcodereview.appspot.com/10831254

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151674 0039d316-1c4b-4281-b951-d872f2087c98