Add a gn implementation for //ios/public/provider/web target.

BUG=459705

Review URL: https://codereview.chromium.org/1443023002

Cr-Commit-Position: refs/heads/master@{#359812}
diff --git a/BUILD.gn b/BUILD.gn
index 5f9f3a9..044f7c6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -205,6 +205,7 @@
   } else {
     deps += [
       "//ios/net:ios_net_unittests",
+      "//ios/public/provider/web",
       "//ios/testing:ocmock_support_unittest",
       "//ios/web:ios_web_unittests",
     ]