Add check_gn_headers.py to find missing headers in GN

testing/buildbot/chromium.linux.json

 {
   "Android Arm64 Builder (dbg)": {
     "additional_compile_targets": [
       "all"
     ]
   },
   "Android Builder": {
     "additional_compile_targets": [
       "cronet_test_instrumentation_apk",
       "system_webview_apk"
@@ skipping 1307 matching lines @@
       },
       {
         "test": "ui_junit_tests"
       },
       {
         "test": "webapk_client_junit_tests"
       },
       {
         "test": "webapk_shell_apk_junit_tests"
       }
+    ],
+    "scripts": [
+      {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      }
     ]
   },
   "Android Tests (dbg)": {
     "gtest_tests": [
       {
         "override_compile_targets": [
           "android_webview_test_apk"
         ],
         "override_isolate_target": "android_webview_test_apk",
         "swarming": {
@@ skipping 1287 matching lines @@
       },
       {
         "test": "ui_junit_tests"
       },
       {
         "test": "webapk_client_junit_tests"
       },
       {
         "test": "webapk_shell_apk_junit_tests"
       }
+    ],
+    "scripts": [
+      {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      }
     ]
   },
   "Blimp Linux (dbg)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
         },
         "test": "blimp_browsertests"
       },
@@ skipping 808 matching lines @@
         "override_compile_targets": [
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       }
     ],
     "scripts": [
       {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      },
+      {
         "name": "checkdeps",
         "script": "checkdeps.py"
       },
       {
         "name": "checkperms",
         "script": "checkperms.py"
       },
       {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
@@ skipping 536 matching lines @@
         "override_compile_targets": [
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       }
     ],
     "scripts": [
       {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      },
+      {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }
     ]
   },
   "Linux Tests (dbg)(1)(32)": {
     "gtest_tests": [
       {
         "swarming": {
           "can_use_on_swarming_builders": true
@@ skipping 347 matching lines @@
         "override_compile_targets": [
           "telemetry_unittests_run"
         ],
         "swarming": {
           "can_use_on_swarming_builders": true
         }
       }
     ]
   }
 }