[ios] Pass more --jscomp_error switches to the closure Compiler.

ios/web/js_compile.gni

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/util/java_action.gni")
 
 declare_args() {
   # Control whether the JavaScript files shipped with Chrome on iOS are
   # compiled with closure_compiler or not. Useful for debugging.
   compile_javascript = true
@@ skipping 95 matching lines @@
     compile_js_target_name = target_name + "_compile_js"
 
     java_action_foreach(compile_js_target_name) {
       visibility = [ ":$_target_name" ]
       script = closure_compiler_path
       sources = invoker.sources
       outputs = [
         "$target_gen_dir/{{source_file_part}}",
       ]
 
-      # TODO(crbug.com/487804): need to enable the following compilation checks
-      # once the corresponding errors have been fixed:
-      #   --jscomp_error=checkTypes
-      #   --jscomp_error=checkVars
-      #   --jscomp_error=missingProperties
-      #   --jscomp_error=undefinedVars
       args = [
         "--compilation_level",
         "SIMPLE_OPTIMIZATIONS",
+        "--jscomp_error=checkTypes",
+        "--jscomp_error=checkVars",
+        "--jscomp_error=missingProperties",
+        "--jscomp_error=undefinedVars",
         "--jscomp_error=accessControls",
         "--jscomp_error=ambiguousFunctionDecl",
         "--jscomp_error=constantProperty",
         "--jscomp_error=deprecated",
         "--jscomp_error=externsValidation",
         "--jscomp_error=globalThis",
         "--jscomp_error=invalidCasts",
         "--jscomp_error=missingReturn",
         "--jscomp_error=nonStandardJsDocs",
         "--jscomp_error=suspiciousCode",
@@ skipping 129 matching lines @@
                                "visibility",
                              ])
 
       sources = invoker.sources
       outputs = [
         "{{bundle_resources_dir}}/{{source_file_part}}",
       ]
     }
   }
 }