Add GN bootstrap step to continuous integration

testing/buildbot/tryserver.chromium.linux.json

 {
   "android_chromium_gn_compile_dbg": {
     "additional_compile_targets": [
       "chrome_shell_apk",
       "mandoline:all"
     ],
     "gtest_tests": []
   },
   "android_chromium_gn_compile_rel": {
     "additional_compile_targets": [
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       },
       {
         "test": "views_unittests"
       },
       {
         "test": "wm_unittests"
       }
     ],
     "scripts": [
       {
+        "name": "gn_bootstrap",
+        "script": "gn_bootstrap.py"
+      },
+      {
         "name": "gn_check",
         "script": "gn_check.py"
       },
       {
         "args": [
           "chrome"
         ],
         "name": "gyp_flag_compare",
         "script": "gyp_flag_compare.py"
       },
       {
         "name": "mojo_apptest",
         "script": "mojo_apptest.py"
       }
     ]
   }
 }