Switch all buildbots to GN build

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.pdfium",
       "--builder",
-      "mac_skia_gn",
+      "linux_skia_gyp",
       "--slave",
       "test_slave",
       "--spec",
       "cache_dir = '[GIT_CACHE]'\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'pdfium', 'url': 'https://pdfium.googlesource.com/pdfium.git'}]",
       "--root",
       "pdfium",
       "--revision_mapping_file",
       "{}",
       "--git-cache-dir",
       "[GIT_CACHE]",
@@ skipping 27 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]/pdfium",
     "env": {
-      "GYP_CHROMIUM_NO_ACTION": "1",
-      "GYP_DEFINES": "pdf_enable_v8=1 pdf_enable_xfa=1 pdf_use_skia=1",
+      "GYP_DEFINES": "pdf_enable_v8=1 pdf_enable_xfa=1 pdf_use_skia=1 target_arch=x64",
       "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=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
   }
 ]