DEPS for gn, used by 'gclient runhooks' to sync gn in buildtools/.

R=machenbach@chromium.org
BUG=chromium:474921
LOG=n

Review URL: https://codereview.chromium.org/1658223002

Cr-Commit-Position: refs/heads/master@{#33690}

DEPS

@@ -136,6 +136,40 @@ hooks = [
                 '-d', 'v8/tools/luci-go/linux64',
     ],
   },
+  # Pull GN using checked-in hashes.
+  {
+    "name": "gn_win",
+    "pattern": ".",
+    "action": [ "download_from_google_storage",
+                "--no_resume",
+                "--platform=win32",
+                "--no_auth",
+                "--bucket", "chromium-gn",
+                "-s", "v8/buildtools/win/gn.exe.sha1",
+    ],
+  },
+  {
+    "name": "gn_mac",
+    "pattern": ".",
+    "action": [ "download_from_google_storage",
+                "--no_resume",
+                "--platform=darwin",
+                "--no_auth",
+                "--bucket", "chromium-gn",
+                "-s", "v8/buildtools/mac/gn.sha1",
+    ],
+  },
+  {
+    "name": "gn_linux",
+    "pattern": ".",
+    "action": [ "download_from_google_storage",
+                "--no_resume",
+                "--platform=linux*",
+                "--no_auth",
+                "--bucket", "chromium-gn",
+                "-s", "v8/buildtools/linux64/gn.sha1",
+    ],
+  },
   {
     # Update the Windows toolchain if necessary.
     'name': 'win_toolchain',