Web UI Polymer3: Fix closure build arguments for JS modules/Polymer 3

- Add a prefix replacement for chrome://resources, since this is needed
across various Web UI pages
- Always add Polymer 3 arguments to closure_flags, even if they are
defined by the invoker. This allows Polymer 3 pages to set additional
prefixes without having to also re-add the polymer 3 closure arguments.

Bug: 965770
Change-Id: Id377a4c454862794ef4330e148615d987bc19030
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742445Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685246}

third_party/closure_compiler/closure_args.gni

@@ -40,6 +40,9 @@ default_closure_args = strict_error_checking_closure_args + [
 
                          "jscomp_off=duplicate",
 
+                         "module_resolution=BROWSER_WITH_TRANSFORMED_PREFIXES",
+                         "browser_resolver_prefix_replacements=\"chrome://resources/=../../ui/webui/resources/\"",
+
                          "js_module_root=../../ui/webui/resources/",
                          "js_module_root=gen/ui/webui/resources/",
                        ]
@@ -54,7 +57,6 @@ polymer3_args = [
   "chrome_pass=false",
   "hide_warnings_for=externs.zip",
   "hide_warnings_for=../../third_party/polymer/v3_0/components-chromium/",
-  "module_resolution=BROWSER_WITH_TRANSFORMED_PREFIXES",
   "polymer_pass=false",
 
   # Note: "2" is counter-intuitively the correct value to use for Polymer 3.

third_party/closure_compiler/compile_js.gni

@@ -221,11 +221,11 @@ template("js_binary") {
     # //third_party/closure_compiler/closure_args.gni
     if (!defined(closure_flags)) {
       closure_flags = default_closure_args
-
-      if (is_polymer3) {
-        closure_flags += polymer3_args
-      }
     }
+    if (is_polymer3) {
+      closure_flags += polymer3_args
+    }
+
     args += [ "--flags" ] + closure_flags
     args += [
       "--externs",