Add gn to DEPS file

Pull the gn binary from google source. This is needed inorder to get git cl
format to work correctly on mac. Added other platforms for completness.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/1826663002 .
diff --git a/DEPS b/DEPS
index 3068d2f..4ca98af 100644
--- a/DEPS
+++ b/DEPS
@@ -69,6 +69,40 @@
     'pattern': '.',
     'action': ['python', 'pdfium/build/gyp_pdfium'],
   },
+  # Pull GN binaries. This needs to be before running GYP below.
+  {
+    'name': 'gn_win',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=win32',
+                '--no_auth',
+                '--bucket', 'chromium-gn',
+                '-s', 'pdfium/buildtools/win/gn.exe.sha1',
+    ],
+  },
+  {
+    'name': 'gn_mac',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=darwin',
+                '--no_auth',
+                '--bucket', 'chromium-gn',
+                '-s', 'pdfium/buildtools/mac/gn.sha1',
+    ],
+  },
+  {
+    'name': 'gn_linux64',
+    'pattern': '.',
+    'action': [ 'download_from_google_storage',
+                '--no_resume',
+                '--platform=linux*',
+                '--no_auth',
+                '--bucket', 'chromium-gn',
+                '-s', 'pdfium/buildtools/linux64/gn.sha1',
+    ],
+  },
   # Pull clang-format binaries using checked-in hashes.
   {
     'name': 'clang_format_win',