Change GetBrowserItemIndex() to GetLauncherItemIndexForType()

Makes GetBrowserItemIndex() more general function to get item index.
Move this function to ./ash/launcher/launcher_item_util.h

R=sky@chromium.org
BUG=NONE
TEST=browser_tests, ash_unittests

Review URL: https://codereview.chromium.org/27369004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230456 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230456