Add cleanup_links.py script that runs before sync.

DEPS

 # This file contains dependencies for WebRTC.
 
 vars = {
   'chromium_git': 'https://chromium.googlesource.com',
   'chromium_revision': '47eb4d6194ddf04274a80779ad35db8722eaee9a',
   'boringssl_git': 'https://boringssl.googlesource.com',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling swarming_client
   # and whatever else without interference from each other.
   'swarming_revision': 'ebc8dab6f8b8d79ec221c94de39a921145abd404',
@@ skipping 120 matching lines @@
     # Dependencies used by libjpeg-turbo
     'src/third_party/yasm/binaries':
       Var('chromium_git') + '/chromium/deps/yasm/binaries.git' + '@' + '52f9b3f4b0aa06da24ef8b123058bb61ee468881',
 
     # WebRTC-only dependency (not present in Chromium).
     'src/third_party/winsdk_samples':
       Var('chromium_git') + '/external/webrtc/deps/third_party/winsdk_samples_v71' + '@' + '6e4f93cc0c2f597547449cbaa80a6af629215a63',
   },
 }
 
+pre_deps_hooks = [
+  {
+    # Remove any symlinks from before 177567c518b121731e507e9b9c4049c4dc96e4c8.
+    # TODO(kjellander): Remove this in March 2017.
+    'name': 'cleanup_links',
+    'pattern': '.',
+    'action': ['python', 'src/cleanup_links.py'],
+  },
+]
+
 hooks = [
   {
     # Check for legacy named top-level dir (named 'trunk').
     'name': 'check_root_dir_name',
     'pattern': '.',
     'action': ['python','-c',
                ('import os,sys;'
                 'script = os.path.join("trunk","check_root_dir.py");'
                 '_ = os.system("%s %s" % (sys.executable,script)) '
                 'if os.path.exists(script) else 0')],
@@ skipping 362 matching lines @@
   },
 ]
 
 recursedeps = [
   # buildtools provides clang_format, libc++, and libc++abi.
   'src/buildtools',
   # android_tools manages the NDK.
   'src/third_party/android_tools',
 ]