Add cleanup_links.py script that runs before sync.

This should reduce/eliminate the amount of manual cleanup users
have to do to sync past 177567c518b121731e507e9b9c4049c4dc96e4c8 (#15754).

This is essentially a stripped down version of
https://chromium.googlesource.com/external/webrtc/+/526248779a6315b02f5ca36a2f76122d3914b67e/setup_links.py
which only does the link cleaning + adds removal of the links db.

BUG=webrtc:5006
NOTRY=True

Review-Url: https://codereview.webrtc.org/2603203002
Cr-Commit-Position: refs/heads/master@{#15897}

DEPS

diff --git a/DEPS b/DEPS
index a12f250..1a502a1 100644
--- a/DEPS
+++ b/DEPS
@@ -138,6 +138,16 @@
   },
 }
 
+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').