android: Fix ARM64 official builds.

https://chromium-review.googlesource.com/c/chromium/src/+/1730834 refactored
some GN files, and as a result generation fails on ARM64, see attached bug.

This is due to an unused variable in some cases. Mark it as such to avoid
generation errors.

TBR: tiborg@chromium.org
Bug: 990278
Change-Id: Ief4463de27cb4afe2699cb18ef0ac8aefca721bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732080Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683533}

chrome/android/modules/chrome_feature_module_tmpl.gni

@@ -137,6 +137,7 @@ template("chrome_feature_module") {
             !build_apk_secondary_abi || android_64bit_target_cpu
         _native_need_placeholder =
             build_apk_secondary_abi && _include_32_bit_webview
+        not_needed([ "_include_32_bit_webview" ])
       } else {
         assert(_loadable_modules_32_bit != [])
         not_needed([