Add hook to download Fuchsia SDK

DEPS

 # This file is used to manage the dependencies of the Chromium src repo. It is
 # used by gclient to determine what version of each dependency to check out, and
 # where.
 #
 # For more information, please refer to the official documentation:
 #   https://sites.google.com/a/chromium.org/dev/developers/how-tos/get-the-code
 #
 # When adding a new dependency, please update the top-level .gitignore file
 # to list the dependency's destination directory.
 #
@@ skipping 1135 matching lines @@
   # Download Telemetry's binary dependencies
   {
     'name': 'fetch_telemetry_binary_dependencies',
     'pattern': '.',
     'action': [ 'python',
                 'src/third_party/catapult/telemetry/bin/fetch_telemetry_binary_dependencies',
     ],
   },
 ]
 
+hooks_os = {
+  'fuchsia': [
+    {
+      'name': 'fuchsia_sdk',
+      'pattern': '.',
+      'action': [
+        'python',
+        'src/build/fuchsia/update_sdk.py',
+        '3886a632c4a472fa7ef9eab35b45b63359e387fa',
+      ],
+    },
+  ],
+  # TODO(scottmg|thakis): Move the Android-only hooks from above here.
+}
+
 recursedeps = [
   # buildtools provides clang_format, libc++, and libc++abi
   'src/buildtools',
   # android_tools manages the NDK.
   'src/third_party/android_tools',
   # ANGLE manages DEPS that it also owns the build files for, such as dEQP.
   ("src/third_party/angle", "DEPS.chromium"),
   # SwiftShader manages DEPS that it also owns the build files for, such as Subzero.
   ("src/third_party/swiftshader", "DEPS"),
 ]