SyncInternals WebUI: Initial attempt to type check JS code.

 - Adding BUILD.gn configuration for JS type checking
 - Fix a lot of type errors
 - Comment out GN targets that still exhibit errors

Repro
 - Add "closure_compile = true" to args.gn
 - ninja -C out/<folder>/ components/sync/driver/resources:closure_compile

Bug: 986001
Change-Id: Ieed34584815b11923c50098d00c072b3da3bb934
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1711211
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680154}

diff --git a/BUILD.gn b/BUILD.gn
index d15e139..7a5ad82e 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -1293,6 +1293,7 @@
     testonly = true
     data_deps = [
       "chrome/browser/resources:closure_compile",
+      "components/sync/driver/resources:closure_compile",
       "components/ukm/debug:closure_compile",
       "content/browser/resources:closure_compile",
       "mojo/public/tools/bindings/generators/js_templates/lite/test:closure_compile",