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

tools/gn/docs/cookbook.md

 # GYP->GN Conversion Cookbook
 
 [TOC]
 
 ## Targets
 
 | *GYP*                                            | *GN*                                               |
 |:-------------------------------------------------|:---------------------------------------------------|
 | `'type': 'static_library', 'name': 'foo',`       | `static_library("foo") {` or `source_set("foo") {` |
 | `'type': 'shared_library', 'name': 'foo',`       | `shared_library("foo") {`                          |
@@ skipping 264 matching lines @@
 | `use_ash` (0/1)                         | `use_ash` (true/false)                         | `//build/config/ui.gni`       |
 | `use_athena` (0/1)                      | `use_athena` (true/false)                      | `//build/config/ui.gni`       |
 | `use_aura` (0/1)                        | `use_aura` (true/false)                        | `//build/config/ui.gni`       |
 | `use_brlapi` (0/1)                      | `use_brlapi` (true/false)                      | `//build/config/features.gni` |
 | `use_cairo` (0/1)                       | `use_cairo` (true/false)                       | `//build/config/ui.gni`       |
 | `use_clipboard_aurax11` (0/1)           | `use_aura && use_x11`                          |                               |
 | `use_cups` (0/1)                        | `use_cups` (true/false)                        | `//build/config/features.gni` |
 | `use_dbus` (0/1)                        | `use_dbus` (true/false)                        | `//build/config/features.gni` |
 | `use_gconf` (0/1)                       | `use_gconf` (true/false)                       | `//build/config/features.gni` |
 | `use_glib` (0/1)                        | `is_linux` (true/false)                        | (global)                      |
-| `use_gnome_keyring` (0/1)               | `is_desktop_linux` (true/false)                |                               |
+| `use_gnome_keyring` (0/1)               | `use_gnome_keyring` (true/false)               |                               |
 | `use_goma` (0/1)                        | `use_goma` (true/false)                        | `//build/toolchain/goma.gni`  |
 | `use_nss_certs` (0/1)                   | `use_nss_certs` (true/false)                   | `//build/config/crypto.gni` (Many of these conditions can be deleted, see the "SSL" notes on targets below.) |
 | `use_pango` (0/1)                       | `use_pango` (true/false)                       | `//build/config/ui.gni`       |
 | `use_ozone` (0/1)                       | `use_ozone` (true/false)                       | `//build/config/ui.gni`       |
 | `use_seccomp_bpf` (0/1)                 | `use_seccomp_bpf` (true/false)                 | `//build/config/features.gni` |
 | `use_udev` (0/1)                        | `use_udev` (true/false)                        | `//build/config/features.gni` |
 | `use_x11` (0/1)                         | `use_x11` (true/false)                         | `//build/config/ui.gni`       |
 | `use_xi2_mt` (0/1)                      | `use_xi2_mt` (true/false)                      | `//build/config/ui.gni`       |
 | `win_use_allocator_shim` (0/1)          |                                                | (See "Allocator" below)       |
 
@@ skipping 379 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```