Add check_gn_headers.py to find missing headers in GN

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 04783b763831654ef4c401dd85cbe8d943be5555..1cf3e5c595d06471d7c7e49a64494713bcd416e8 100644
--- a/testing/buildbot/chromium.linux.json
+++ b/testing/buildbot/chromium.linux.json
@@ -1325,6 +1325,12 @@
       {
         "test": "webapk_shell_apk_junit_tests"
       }
+    ],
+    "scripts": [
+      {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      }
     ]
   },
   "Android Tests (dbg)": {
@@ -2632,6 +2638,12 @@
       {
         "test": "webapk_shell_apk_junit_tests"
       }
+    ],
+    "scripts": [
+      {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      }
     ]
   },
   "Blimp Linux (dbg)": {
@@ -3460,6 +3472,10 @@
     ],
     "scripts": [
       {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      },
+      {
         "name": "checkdeps",
         "script": "checkdeps.py"
       },
@@ -4016,6 +4032,10 @@
     ],
     "scripts": [
       {
+        "name": "check_gn_headers",
+        "script": "check_gn_headers.py"
+      },
+      {
         "name": "nacl_integration",
         "script": "nacl_integration.py"
       }