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}

• BUILD.gn[diff]
• components/sync/driver/resources/BUILD.gn[Added - diff]
• components/sync/driver/resources/chrome_sync.js[diff]
• components/sync/driver/resources/data.js[diff]
• components/sync/driver/resources/index.html[diff]
• components/sync/driver/resources/search.js[diff]
• components/sync/driver/resources/sync_log.js[diff]
• components/sync/driver/resources/sync_node_browser.js[diff]
• components/sync/driver/resources/sync_search.js[diff]
• components/sync/driver/resources/traffic_log.js[diff]
• components/sync/driver/resources/types.js[diff]