Re-enabling libyuv 'gn check'.

Libyuv is now 'gn check' compatible and the fixed version has been
rolled into chromium (r1697).

Bug: webrtc:8850
Change-Id: Iaaeae229571fd02045322c4f8addadd75f889bdb
No-Try: True
Reviewed-on: https://webrtc-review.googlesource.com/50180
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21966}

.gn

@@ -46,9 +46,7 @@ check_targets = [
   "//system_wrappers/*",
   "//test/*",
   "//video/*",
-
-  # TODO(bugs.webrtc.org/8850): re-enable when libyuv 'gn check' will be fixed.
-  # "//third_party/libyuv/*",
+  "//third_party/libyuv/*",
 ]
 
 # These are the list of GN files that run exec_script. This whitelist exists