Add a windows_xfa_clang trybot and enable clang_use_chrome_plugins

scripts/slave/recipes/pdfium.expected/linux_skia_gn.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.pdfium",
       "--builder",
       "linux_skia_gn",
@@ skipping 51 matching lines @@
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gn.py",
       "--root=[SLAVE_BUILD]/pdfium",
       "gen",
       "//out/Debug",
-      "--args=is_debug=true pdf_enable_v8=true pdf_enable_xfa=true pdf_use_skia=true pdf_is_standalone=true clang_use_chrome_plugins=false"
+      "--args=is_debug=true pdf_enable_v8=true pdf_enable_xfa=true pdf_use_skia=true pdf_is_standalone=true"
     ],
     "cwd": "[SLAVE_BUILD]/pdfium",
     "name": "gn gen"
   },
   {
     "cmd": [
       "ninja",
       "-C",
       "[SLAVE_BUILD]/pdfium/out/Debug"
     ],
     "name": "compile with ninja"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]