Add bug reference on //third_party/webrtc/* entry in .gn.

TBR: jochen
Bug: 812058
Change-Id: I36f9060adf3c8255cee85d688aab0d26c75819c2
Reviewed-on: https://chromium-review.googlesource.com/976046Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545194}

.gn

@@ -161,7 +161,7 @@ check_targets = [
   "//third_party/snappy/*",
 
   #"//third_party/WebKit/*",  # Errors: https://crbug.com/800764
-  #"//third_party/webrtc/*",
+  #"//third_party/webrtc/*",  # Errors: https://crbug.com/824831
   "//third_party/webrtc_overrides/*",
   "//tools/*",