WebUI: Clean up more closure compiler references

- Fix a potential build issue by ensuring builds that manually override
  enable_js_type_check only add the webui_closure_compile target on
  ChromeOS Ash.
- Update documentation to indicate closure should never be used on other
  platforms.

Bug: 1316438
Change-Id: I44d35f9805ca56d3825a3c4106964740bdec5b7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4185395
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1095761}

BUILD.gn

@@ -115,7 +115,7 @@ group("gn_all") {
     deps += [ "//third_party/abseil-cpp:absl_tests" ]
   }
 
-  if (enable_js_type_check) {
+  if (is_chromeos_ash && enable_js_type_check) {
     deps += [ ":webui_closure_compile" ]
   }
 

docs/closure_compilation.md

@@ -1,5 +1,9 @@
 # Closure Compilation
 
+**Important: Closure Compilation is only supported on ChromeOS Ash. On all
+other platforms, Closure Compiler is deprecated; TypeScript should be used
+for type checking.** See [bug](https://www.crbug.com/1316438)
+
 ## What is type safety?
 
 [Statically-typed languages](https://en.wikipedia.org/wiki/Type_system#Static_type_checking)

styleguide/web/web.md

@@ -341,10 +341,10 @@ if (!enterKey) {
   feature when the code needs to handle null/undefined gracefully.
 
 
-### Closure compiler
+### Closure compiler (ChromeOS Ash code only)
 
-* Closure compiler should only be used by legacy code that has not yet been
-  converted to use TypeScript.
+* Closure compiler can only be used on ChromeOS Ash. All other platforms
+  are required to use TypeScript to add type checking.
 
 * Use the [closure
   compiler](https://chromium.googlesource.com/chromium/src/+/main/docs/closure_compilation.md)