Enable "gn check" on //third_party/webrtc.

Problems have been fixed by:
- https://webrtc-review.googlesource.com/97620
- https://webrtc-review.googlesource.com/97622

Bug: 824831
Change-Id: Ibe2f12f6d935c61a37cd253d22960a1ccba9f50a
Reviewed-on: https://chromium-review.googlesource.com/1203773Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590016}

.gn

@@ -328,7 +328,7 @@ check_targets = [
   "//third_party/snappy/*",
 
   #"//third_party/WebKit/*",  # Errors: https://crbug.com/800764
-  #"//third_party/webrtc/*",  # Errors: https://crbug.com/824831
+  "//third_party/webrtc/*",
   "//third_party/webrtc_overrides/*",
   "//tools/*",