[Mac] Use an AppleEvent to tell the Finder to open downloaded items, rather than NSWorkspace.

BUG=32921,50263
TEST=Force a PDF to download. Quit Preview, if open. Open the downloaded PDF from the download shelf. Preview opens and becomes frontmost.
TEST=Download a file of a type that you do not have an application with which to open it. Open it from the download shelf. Finder bounces for your attention to choose an application to open it.

Review URL: http://codereview.chromium.org/3151011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@56026 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed