Switch all buildbots to GN build

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.pdfium",
       "--builder",
       "linux_xfa",
@@ skipping 37 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]/pdfium",
     "env": {
+      "GYP_PDFIUM_NO_ACTION": "1",
       "GYP_DEFINES": "pdf_enable_v8=1 pdf_enable_xfa=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "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=false pdf_is_standalone=true"
+    ],
+    "cwd": "[SLAVE_BUILD]/pdfium",
+    "name": "gn gen"
+  },
+  {
+    "cmd": [
       "ninja",
       "-C",
       "[SLAVE_BUILD]/pdfium/out/Debug"
     ],
     "name": "compile with ninja"
   },
   {
     "cmd": [
       "[SLAVE_BUILD]/pdfium/out/Debug/pdfium_unittests"
     ],
@@ skipping 38 matching lines @@
       "out/Debug"
     ],
     "cwd": "[SLAVE_BUILD]/pdfium",
     "name": "corpus tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
-]
+]