Add gn to DEPS file

DEPS

 use_relative_paths = True
 
 vars = {
   'chromium_git': 'https://chromium.googlesource.com',
   'pdfium_git': 'https://pdfium.googlesource.com',
 
   'buildtools_revision': 'c2f259809d5ede3275df5ea0842f0431990c4f98',
   'cygwin_revision': 'c89e446b273697fadf3a10ff1007a97c0b7de6df',
   'gmock_revision': '29763965ab52f24565299976b936d1265cb6a271',
   'gtest_revision': '8245545b6dc9c4703e6496d1efd19e975ad2b038',
@@ skipping 51 matching lines @@
   '+third_party/base',
 ]
 
 hooks = [
   {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     'name': 'gyp',
     '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',
     'pattern': '.',
     'action': [ 'download_from_google_storage',
                 '--no_resume',
                 '--platform=win32',
                 '--no_auth',
                 '--bucket', 'chromium-clang-format',
                 '-s', 'pdfium/buildtools/win/clang-format.exe.sha1',
@@ skipping 21 matching lines @@
                 '-s', 'pdfium/buildtools/linux64/clang-format.sha1',
     ],
   },
   {
     # Pull clang if needed or requested via GYP_DEFINES.
     'name': 'clang',
     'pattern': '.',
     'action': ['python', 'pdfium/tools/clang/scripts/update.py', '--if-needed'],
   },
 ]