Re-commit, reverted in http://crrev.com/157680 (original commit in http://crrev.com/157667).

This allows usage of PathService to cache the shortcut install paths and more importantly to mock them in shortcut tests!

Also move chrome::DIR_USER_DESKTOP to base::DIR_USER_DESKTOP; this is really where it belongs. In fact it is only in chrome_paths.h because it used to be called DIR_DEFAULT_DOWNLOAD and cpu@ renamed it to DIR_USER_DESKTOP in http://crrev.com/1753 (early days!) after that it started to be used all over the place as the Desktop path. Finally bringing it to base_paths.h, beside DIR_START_MENU and friends, is the right thing to do imo.

TBR=brettw@chromium.org
BUG=148539
TEST=Quick Launch shortcut installed in the right place on XP (both Default and current user)
Desktop shortcuts installed in the right place (both All Users and per-user installs).

installer_util_unittests.exe --gtest_filter=ShellUtilShortcutTest*
unit_tests.exe --gtest_filter=ProfileShortcutManagerTest*
base_unittests --gtest_filter=PathServiceTest*

Review URL: https://chromiumcodereview.appspot.com/10964007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@157779 0039d316-1c4b-4281-b951-d872f2087c98