[Mac] Enable Content-Disposition: attachment sandbox

[Mac] Enable Content-Disposition: attachment sandbox
https://bugs.webkit.org/show_bug.cgi?id=155578
<rdar://problem/21886326>

Reviewed by Dan Bernstein.

Covered by the existing set of attachment sandbox tests, which have always been run on Mac.

Source/WebKit/mac:

* WebView/WebView.mm:
(-[WebView _commonInitializationWithFrameName:groupName:]):

Source/WebKit2:

* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::WebPage):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@198318 268f45cc-cd09-0410-ab3c-d52691b4dbfc