Add script tests for `gn check`, `gyp_flag_compare` to GN bot configs.

testing/buildbot/chromium.linux.json

Index: testing/buildbot/chromium.linux.json
diff --git a/testing/buildbot/chromium.linux.json b/testing/buildbot/chromium.linux.json
index c18aece41b8a1b9f3f75f45c1487b5312fdc59e3..4f9452c8921563e646f4bf5c8435811e5e3871af 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -3,6 +3,12 @@
     "additional_compile_targets": [
       "chrome_shell_apk",
       "mandoline:all"
+    ],
+    "scripts": [
+      {
+        "name": "gn_check",
+        "script": "gn_check.py"
+      }
     ]
   },
   "Android GN (dbg)": {
@@ -306,6 +312,19 @@
       {
         "test": "wm_unittests"
       }
+    ],
+    "scripts": [
+      {
+        "name": "gn_check",
+        "script": "gn_check.py"
+      },
+      {
+        "args": [
+          "chrome"
+        ],
+        "name": "gyp_flag_compare",
+        "script": "gyp_flag_compare.py"
+      }
     ]
   },
   "Linux GN (dbg)": {