gn: Use a separate flag for enabling libgnome-keyring support.

Right now, |is_desktop_linux| means GN requires libgnome-keyring to
be present during the build and also at run-time.

Since libgnome-keyring has been deprecated by libsecret, is used by a
small number of users and is not always present on a Linux
distribution (especially embedded ones), it makes sense to introduce a
specific argument, |use_gnome_keyring|, to control whether the code
should be used or not.

At the moment, |use_gnome_keyring| defaults to on under pretty much all
circumstances where checking for |is_desktop_linux| would work before:
it just needs |use_glib| to be on as well, as required by
libgnome-keyring itself.

R=brettw@chromium.org,vabr@chromium.org,thestig@chromium.org,jochen@chromium.org
BUG=466975,602624

Review-Url: https://codereview.chromium.org/2244653002
Cr-Commit-Position: refs/heads/master@{#412246}
5 files changed