[Mac] On Mavericks or later, use NSWorkspace to open download items.

With the NSWorkspaceLaunchWithErrorPresentation option, Finder no longer
silently fails opening the file.

BUG=32921
R=avi@chromium.org
TEST=Download a DMG file. Click the download shelf button to open/mount it. Finder activates the new disk image window as the topmost.
TEST=Download a file with an unassociated file extension type (e.g.  "test.foobar"). Click the download shelf button to open it. Finder presents the unknown file type dialog.

Review URL: https://codereview.chromium.org/613673003

Cr-Commit-Position: refs/heads/master@{#297239}
2 files changed