Adding video_processing to presubmit lint check

BUG=webrtc:5259
R=pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1507213002 .

Cr-Commit-Position: refs/heads/master@{#10928}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 2c0da0f..c803542 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -18,6 +18,7 @@
 CPPLINT_DIRS = [
   'webrtc/audio',
   'webrtc/call',
+  'webrtc/modules/video_processing',
 ]
 
 # List of directories of "supported" native APIs. That means changes to headers