[AW] Enable gn check for //android_webview/*

Bug: 934152
Change-Id: Iee469873ec66edaf1f497317cbdf686eb74ec6ee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1832916
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701836}

.gn

@@ -71,6 +71,7 @@ default_args = {
 # their includes checked for proper dependencies when you run either
 # "gn check" or "gn gen --check".
 check_targets = [
+  "//android_webview/*",
   "//apps/*",
   "//ash/*",
   "//base/*",