Add //webrunner to GN dependency checking, and remove //win8.

Change-Id: Id2debad3159b74fe50a89368aa940d686a0f015b
Reviewed-on: https://chromium-review.googlesource.com/990081
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547550}

.gn

@@ -169,7 +169,7 @@ check_targets = [
   "//url/*",
 
   #"//v8/*",  # Errors: https://bugs.chromium.org/p/v8/issues/detail?id=7330
-  "//win8/*",
+  "//webrunner/*",
 ]
 
 # These are the list of GN files that run exec_script. This whitelist exists