Add a DEPS gclient hook to prune corrupt mockito remote.

A third_party remote for third_party/mockito/src is corrupt. Run
`git remote prune origin` in the checkout directory before syncing
Chromium.

Thanks to slan@ for providing this fix.

TBR=kjellander@webrtc.org
BUG=chromium:644722
NOTRY=True

Review-Url: https://codereview.webrtc.org/2326523002
Cr-Commit-Position: refs/heads/master@{#14138}

sync_chromium.py

@@ -31,7 +31,7 @@ import textwrap
 
 # Bump this whenever the algorithm changes and you need bots/devs to re-sync,
 # ignoring the .last_sync_chromium file
-SCRIPT_VERSION = 8
+SCRIPT_VERSION = 9
 
 ROOT_DIR = os.path.dirname(os.path.abspath(__file__))
 CHROMIUM_NO_HISTORY = 'CHROMIUM_NO_HISTORY'