[ios] Silence warning from closure compiler until dep can be rolled

The warning is benign (warn about an empty block protected by an if)
but cause Xcode 12.0+ to consider the build as a failure (as soon as
the output contains the string "error", Xcode consider the build as
a failure).

Silence the warning until the dependency can be rolled (currently
blocked by the fact that mirroring from github is misconfigured).

Bug: 1129390, 1129393
Change-Id: I170de8cac0fa88612bac157737f7abd2fbfa4989
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2416273
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807904}

ios/web/BUILD.gn

@@ -583,6 +583,10 @@ js_compile_bundle("main_frame_web_bundle") {
   visibility = [ ":js_resources" ]
   closure_entry_point = "__crWeb.mainFrameWebBundle"
 
+  # Silence warning when compiling text-fragment-utils.js until the code
+  # has been rolled (see https://crbug.com/1129390).
+  suppressed_warnings = [ "uselessCode" ]
+
   sources = [
     "//third_party/text-fragments-polyfill/src/src/text-fragment-utils.js",
     "web_state/js/resources/error.js",

ios/web/js_compile.gni

@@ -24,6 +24,9 @@ closure_compiler_path = "//third_party/closure_compiler/compiler/compiler.jar"
 #   deps (optional)
 #     List of targets required by this target.
 #
+#   suppressed_warnings (optional)
+#     List of warnings to suppress.
+#
 #   visibility (optional)
 #     Visibility restrictions.
 #
@@ -49,7 +52,14 @@ template("js_compile_bundle") {
     sources = invoker.sources
     outputs = [ "$target_gen_dir/$_target_name.js" ]
 
-    args = [
+    _additional_args = []
+    if (defined(invoker.suppressed_warnings)) {
+      foreach(_suppressed_warning, invoker.suppressed_warnings) {
+        _additional_args += [ "--jscomp_off=$_suppressed_warning" ]
+      }
+    }
+
+    args = _additional_args + [
              "--compilation_level",
              "SIMPLE_OPTIMIZATIONS",
              "--js_output_file",