Include //third_party/WebKit/Source/* in default gn check targets.

It checks cleanly, even though //third_party/WebKit/public does not.

Review URL: https://codereview.chromium.org/1491333002

Cr-Commit-Position: refs/heads/master@{#363027}

.gn

@@ -82,6 +82,7 @@ check_targets = [
   "//testing/*",
 
   #"//third_party/*",  # May not ever want this.
+  "//third_party/WebKit/Source/*",
   "//tools/*",
 
   #"//ui/*",  # Just a few problems.