Move `gn check` into the `generate_build_files` step.

testing/buildbot/tryserver.chromium.linux.json

 {
   "android_chromium_gn_compile_dbg": {
     "additional_compile_targets": [
       "blimp",
       "chrome_public_apk",
       "chrome_public_test_apk",
       "content_shell_apk",
       "mandoline:all"
     ]
   },
@@ skipping 329 matching lines @@
       },
       {
         "test": "views_unittests"
       },
       {
         "test": "wm_unittests"
       }
     ],
     "scripts": [
       {
-        "name": "gn_check",
-        "script": "gn_check.py"
-      },
-      {
         "args": [
           "chrome"
         ],
         "name": "gyp_flag_compare",
         "script": "gyp_flag_compare.py"
       },
       {
         "name": "mojo_apptests",
         "script": "mojo_apptests.py"
       }
     ]
   }
 }